Merge 20a0fde Tidying up default clipboard handler for non Windows-OS (from Nav branch)

Merge 2545d75 from Nav - Tidying up, removed two unnecessary window flags from being exposed in imgui.h

Minor miscellaneous merges from Navigation branch to reduce divergence a little bit

ImRect::Clip() -> ClipWith() to clarify meaning, going to undo the change in Nav branch.

Fixed an assert when calling CloseCurrentPopup() twice in a row.

Removed tabs that slipped through the cracks

Columns: Caching columns clipping rectangles (#125)

Columns: Moved PushColumnClipRect() to imgui_internal.h

Columns: Fix for explicit content width and scrollbar. (#519, #125)

Columns: Columns set resize based on the presence of vertical scrollbar. Not 100% sure about that but it looks like we've fixed enough bugs that this may not cause troubles anymore. (#125, #913, #893, #1138)

Demo: Columns: Added Horizontal Scrolling demo. Tweaked another Columns demo. (#519, #125, #913)

Columns: Fixed another form of shearing made visible by the PreserveWidth option. The code was initially added in c46d563 to fix because we've fixed in e42aaed. (#913, #125)

Columns: A set of column (and most importantly the right-most column) do not register its content to the parent, not affecting the window contents size. (#519, #125, #913)

Columns: Added ImGuiColumnsFlags_NoResize flag (internal). (#913, #125)

Columns: Fixed one form of shearing during resizing columns with PreserveWidth enabled (there's another issue still). (#913, #125)

Columns: Removed unnecessary/misleading dummy ItemSize(). Was fixed already before, and fixed again in this branch by 3bf2af2. (#913, #125). End() calls EndColumns() directly.

Columns: Fixed offset rounding leading to SetColumnOffset() being destructive when ImGuiColumnsFlags_NoPreserveWidths flag is not set. (#913, #125)

Columns: Fixed EndColumns() not repositioning the cursor. (#913)

Columns: Moved BeginColumns/EndColumns/flags from #913 to imgui_internals.h + minor shallow tweaks. Removed demo code temporarily. (#125)

Merge branch 'master' of https://github.com/ggtucker/imgui into ggtucker-master ...

# Conflicts:
#	imgui.cpp
#	imgui.h

Renamed GetStyleColName() to GetStyleColorName() for consistency. Extra comments in Api Breaking Changes section.

Added GetStyleColorVec4()

GCC warnings fixes

Scrollbar: Comments. Fixed potential div-by-zero error which I can repro now, so added an assert to notify us if it's actually ever useful or not.

Added PushStyleColor(ImGuiCol idx, ImU32 col) overload. (Which _might_ cause an "ambiguous call" compilation error if you are using ImColor() with implicit cast. Cast to ImU32 or ImVec4 explicily to fix.)

Merge branch 'master' into styles ...

# Conflicts:
#	TODO.txt

Marked the weird IMGUI_ONCE_UPON_A_FRAME helper macro as obsolete. prefer using the more explicit ImGuiOnceUponAFrame. Removed the broken __LINE__ from IMGUI_ONCE_UPON_A_FRAME

OpenPopupEx() internal tweaks to receive an ImGuiID, BeginPopupContextXXX shortening unnecessarily long identifier.

PushID()/PopID() to not need to mark parent window as Accessed (needlessly waking up the root "Debug" window) (#747)

(api breaking) changed parameter order for BeginPopupContextWindow(), note that most uses relied on default parameters completely.