Comments, minor tweaks

Added IMGUI_CHECKVERSION() macro to compare version string and data structure sizes in order to catch issues with mismatching compilation unit settings. (#1695, #1769)

Misc: Comments and shallow/small changes (merged from viewport branch to minimize branch drift).

Fix various typos.

BeginMainMenuBar: Followup to 1e41bad9. Removed public window flag. DisplaySafeAreaPadding defaults to (3,3) instead of (4.4). Comments, tab to spaces, Changelog. (#1439)

Added DisplaySafeAreaPadding to MainMenuBar height and fixed starting x position. Fixes main menu bar problems with TVs. (#1439)

Fix typos KeyDown => KeysDown

Misc: IM_DELETE() helper function added in 1.60 doesn't clear the input _pointer_ reference, more consistent with expectation and allows passing r-value.

Version 1.61 WIP

Version 1.60 (missed the string). Will retag.

Nav: Fixed comment. (#1599)

Style: Default style is now StyleColorsDark()! Toward a 1.60 release :) (#707)

Comments about io.WantCapture flags + todo entries.

IO: Added ImGuiConfigFlags_NoMouse to force clear/disable mouse inputs in NewFrame().

Renamed ImGuiConfigFlags_NoSetMouseCursor to ImGuiConfigFlags_NoMouseCursorChange. Followup to 75c3793 two weeks ago. (#787, #1495, #1202) + comments

Added extra comments and assertions to avoid user combining ImGuiCond flags. (#1694)

Tooltip windows uses PopupBorderSize (#1697) + commented out debug stuff.

Added InputDouble() function. We use a format string instead of a decimal_precision parameter to also for "%e" and variants. (#1011) May transition the other InputXXX function to use format strings as well.

InputFloat: Scientific input. InputText: Added ImGuiInputTextFlags_CharsScientific to add 'e' 'E' to list of characters that can be input. (later useful for #1011)

IO: Added ImGuiConfigFlags_NoSetMouseCursors. Added ImGuiBackendFlags_HasMouseCursors, ImGuiBackendFlags_HasSetMousePos. (#787, #1495, #1202)

Added ImGuiBackendFlags for backend to expose its capabilities. Added ImGuiBackendFlags_HasGamepad. (#787, #1542)

IO: Renamed io.WantMoveMouse to io.WantSetMousePos (was added in 1.52, _not_ used by core and only honored by some binding ahead of merging the Nav branch) + internal renaming (#787)

ImVector: Fixed insert() helper using the = operator (followup to be consistent with 4186c2c)

Nav: Track nav input source more generally (gamepad vs keyboard) (#787) + update todos and demo tweaks

imgui.h: Various comments and tweaks.

imgui.h: Moved columns lower in the file. Various spacing changes and minor comments.

ImVector: Added copy/assign operators as helpers to ease cloning of data.

ImDrawList: Added Clone() helper function.

ImVector: resize() / push_back() use memcpy() as we are not supposed to be constructor/destructor/operator aware at all. (If we do need to rework ImVector). In turn this will allow us to define assign/copy operators.

ImVector: Using value_type consistently. Spacing.