TODO update + internals: changed order or ImGuiLayoutType enums to allow using them for indexing.

Merge branch 'atlas_fixes'

FAQ entry, moved ImTextureId, Gallery links.

ImFontAtlas: Rewrote stb_truetype based builder. ...

- Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth).
- Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233)
- Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas.

ImFontGlyphRangesBuilder: Using 32-bits fields for storage instead of 8-bit ones, comments, todo.

Demo: Log: Comments. Using clipper. Not linking with rand() anymore.

Fixed using SetNextWindowPos() on a child window (which wasn't really documented) position the cursor as expected in the parent window, so there is no mismatch between the layout in parent and the position of the child window. Demo tweak and adding some child window stuff

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.

Version 1.67 WIP + todo notes

Refactor: Moved README, ChangeLog, TODO files to docs/ folder + update Changelog. (#2036)