Refactor: Move viewport code under other subsystem to simplify merging (1) (moving in multiple commits to make diff/patch behave nicely)

Internal: Columns: Allow to use BeginColumns(1) so code designed for variable number of columns can still call NextColumn etc. (#125)

TabBar: Fixed ImGuiTabItemFlags_SetSelected being ignored if the tab is not visible (with scrolling policy enabled) or if is currently appearing.

Made ImS8 and ImS16 explicitly signed in case some crazy SDK decide to flip the signedness over. (#2408)

Fix for Android char being unsigned by default (#2408)

Examples: DirectX9: Added support for multi-viewport (#2394)

Merge branch 'master' into docking ...

# Conflicts:
#	imgui_demo.cpp

Internals: ColorEdit: Minor optimizations. Initialize internal arrays as static const, avoid unnecessary HSV->RGB conversion.

Demo: Renamed ShowHelpMarker() -> HelpMarker(). Fixed minor PVS warning. Removed unnecessary casts.

Examples: DirectX9: Minor changes to match the other DirectX examples more closely. (#2394)

InputTextWithHint: Fix for Password fields. Update changelog, demo. (#2400)

Added InputTextWithHint() (#2400) ...

Squashed commit of the following:

commit 1970d84
Author: Lucas Lazare <lazarelucas@yahoo.fr>
Date:   Tue Mar 5 12:20:39 2019 -0500

    Removing sneaky tabulations #2 (why, editor T-T)

    I should update my settings, I guess

commit 219bdfc
Author: Lucas Lazare <lazarelucas@yahoo.fr>
Date:   Tue Mar 5 12:17:27 2019 -0500

    Removing useless check introduced in b0d172

commit 8afd7a2
Author: Lucas Lazare <lazarelucas@yahoo.fr>
Date:   Tue Mar 5 11:49:24 2019 -0500

    Removing sneaky tabulations

commit 8e04908
Author: Lucas Lazare <lazarelucas@yahoo.fr>
Date:   Tue Mar 5 11:45:13 2019 -0500

    Moving InputTextHinted code to InputTextEx

commit b0d1723
Author: Lucas Lazare <lazarelucas@yahoo.fr>
Date:   Tue Mar 5 00:23:02 2019 -0500

    C++11 to C++98

commit 9afeae3
Author: Lucas Lazare <lazarelucas@yahoo.fr>
Date:   Mon Mar 4 23:43:28 2019 -0500

    Added InputTextHinted

InputText: Moving some code in anticipation of supporting hint display with password. This commit is aimed at having no visible side effect. (#2400)

Removed redundant declaration of SetNextWindowClass. (#2402)

Merge branch 'master' into docking ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Fixed IsItemDeactivated()/IsItemDeactivatedAfterEdit() from not correctly returning true when tabbing out of a focusable widget (Input/Slider/Drag) in most situations. (#2215, #1875) ...

+ Minor renaming of a local variable in widget code.

Internal: Tabbing/Focus: Tidying up old code, moved some state to context instead of window. Storing new data will allow us to fix the bug mentioned in #2215 (probably in next commit).

Demo: Added flags to InputTextMulttiline() demo.

MenuItem, Selectable: Fixed disabled widget interfering with navigation (fix c2db7f63 in 1.67).

Internal: Tabbing: Tweaks to FocusableItemRegister and using the standard mechanism to allow/block Tab being interpreting by tabbing instead of InputText() widget.

Internal: Text: Extracted TextUnformatted into TextEx over which we can freely atter the signature. Clarified current large text behavior of TextUnformatted with explicit ImGuiTextFlags_NoWidthForLargeClippedText flag (always set).

Text: Fixed large Text/TextUnformatted call not declaring its size when starting below the lower point of the current clipping rectangle. Somehow this bug has been there since v1.0! It was hardly noticeable but would affect the scrolling range, which in turn would affect some scrolling request functions when called during the opening frame of a window.

Nav: Fixed Ctrl+Tab keeping active InputText() of a previous window active after the switch. (#2380)

Comments and Issue Template

Merge branch 'master' into docking ...

# Conflicts:
#	imgui.cpp
#	imgui.h
#	imgui_internal.h

Renamed GetOverlayDrawList() to GetForegroundDrawList() for consistency. Kept redirection function (will obsolete). (#2391) ...

Demo: Using GetBackgroundDrawList() and GetForegroundDrawList() in "Custom Rendering" demo.

Added GetBackgroundDrawList() helper to quickly get access to a ImDrawList that will be rendered behind every other windows. (#2391)

Merge branch 'master' into docking ...

# Conflicts:
#	examples/imgui_impl_opengl3.cpp

Examples: OpenGL: Fix to be able to run on ES 2.0 / WebGL 1.0. [@rmitton, @gabrielcuvillier]

Examples: OpenGL: Comments about versions and loaders. (#2393, #2351)