Added BETA api for Tab Bar/Tabs widgets. (#261, #351) (merged this feature from the from Docking branch so it can be used earlier as as standalone feature) ...

- Added BeginTabBar(), EndTabBar(), BeginTabItem(), EndTabItem(), SetTabItemClosed() API.
- Added ImGuiTabBarFlags flags for BeginTabBar().
- Added ImGuiTabItemFlags flags for BeginTabItem().
- Style: Added ImGuiCol_Tab, ImGuiCol_TabHovered, ImGuiCol_TabActive, ImGuiCol_TabUnfocused, ImGuiCol_TabUnfocusedActive colors.
- Demo: Added Layout->Tabs demo code.
- Demo: Added "Documents" example app showcasing possible use for tabs.

Using named flags instead of 0 + shallow formatting tweaks from other branches.

Tests: Adding imgui-test engine hooks (experimental) to provide missing widget state to the testing system.

Internals: Nav: Added ImGuiNavLayer_ to clarify semantic of previously integer NavLayer values, and not pretend that increment/decrement operators on them super flexible. + Storage tweaks.

Version 1.67 WIP (again, this time IMGUI_VERSION_NUM has leeway for another hot-fix).

Version 1.66b (will revisit how to change IMGUI_VERSION_NUM across versions, this commit reduces the numerical IMGUI_VERSION_NUM compared to the commit on Nov 22).

When the focused window become inactive don't restore focus to a window with the ImGuiWindowFlags_NoInputs flag. (#2213)

Internals: Added ImPool helper data structure (used by testing system, and already in docking branch)

Version 1.67 WIP

Version 1.66

Internals: Added FindWindowByID() helper. (#2190)

Nav: Fixed an assert in certain circumstance (mostly when using popups) when mouse positions stop being valid. (#2168) + adding a else block to make NavCalcPreferredRefPos() more explicit.

Internals: DragBehavior: Added support for ImGuiDragFlags_Vertical to implement a vertical drag widget (no frontend function provided).

Comments in demo and opengl code + Internals: Added HoveredIdNotActiveTimer tracking hovering time unless the item is active, which is a commonly useful pattern.

Shutdown: Unlock font atlas before destroying context, so we can destroy a context between NewFrame and EndFrame if we wait (facilitate main loop structures). Internals: GetWindowScrollMaxX(), GetWindowScrollMaxY()

TextUnformatted: Using memchr(), fixed not properly testing for text_end bound + comments. ...

Internals: Added ImStreolRange() + used in LogRenderedText() + comments.

Comments + internal using Tab Stop terminology (ImGuiItemFlags_NoTabStop instead of !ImGuiItemFlags_AllowKeyboardFocus)

Nav, Focus: Fixed ImGuiWindowFlags_NoBringToFrontOnFocus windows not being restoring focus properly after the main menu bar or last focused window is deactivated.

Version 1.66 WIP

Version 1.65

Renamed stb_truetype.h to imstb_truetype.h, stb_textedit.h to imstb_textedit.h, and stb_rect_pack.h to imstb_rectpack.h. (#1718, #2036) ...

If you were conveniently using the imgui copy of those STB headers in your project, you will have to update your include paths.
The reason for this change is to avoid conflicts for projects that may also be importing their own copy of the STB libraries. Note that imgui's copy of stb_textedit.h is modified.

Minor mostly inconsequential merges from Viewport + LoadIniSettingsFromMemory() entirely skip lines starting with ';'

Internals: Exposed SliderBehaviorT, DragBehaviorT, RoundScalarWithFormatT, SliderCalcRatioFromValueT. (#2036) ...

Renamed RoundScalarWithFormat -> RoundScalarWithFormatT.
Renamed SliderBehaviorCalcRatioFromValue -> SliderCalcRatioFromValueT

Version 1.65 WIP

Version 1.64

Refactor: Internals: Moved Settings functions in imgui.cpp in their own section. (#2036)

Version 1.64 WIP

Version 1.63

Internals: Tidying up, moving helpers to imgui_internal.h (in prevision of 1.64 refactor)

Internals: Merged some changes to facilitate 1D->2D transition for some elements, and merging the stack layout PR (#846)