MenuBar clipping rectangle fix for bordered mode

Various adjustment for border to be inside instead of outside the window size

Fixed SizeContents and auto-fit size not being rounded to integer, which affects border rendering when using non-default font

InputText: update mouse drag on mouse move only & key input reposition selection bounds (better fix for #429)

InputText: when keys are being input while mouse is dragging, disable mouse drag until button release (#429)

Metrics: computing drawcmd bounding box handle case where buffer has been de-indexed.

Minor tidying up of now redundant code

Renamed Color() helpers to ValueColor() - dangerously named, rarely used and probably to be made obsolete.

Added SetItemAllowOverlap() exposed in imgui_internal.h only

Commenting asserts

TreeNode/CollapsingHeader: some minor cleanup of that horrible function.

TreeNode/CollapsingHeader: some minor cleanup of that horrible function.

Comments & spacing

Added comment on using PushFont() before building the font (#232)

TreeNode,CollapsingHeader: sizing fixes to support laying out item on same line before/after (#414, #282)

Internal minor renaming

Replace OS IME (Input Method Editor) cursor on top-left when we are not text editing.

Examples: Clarified comments. Removed font merging example from comments.

Additional comments for new users

Demo: comments (#414)

Columns: Fixed bug with indentation within columns, also making code shorter/faster (#414, #125)

Columns: Undo accidental change from 807d4ff

Columns:: columns set with no implicit id include the columns count into the id to avoid collisions (#125)

Columns: minor tidying up while investigating severe issues with indenting within columns (#414)

Columns: avoid growing ColumnsData vector when closing a window with no columns (#125) ...

(Will break calling GetColumnOffset() when there's no column but it
would only have worked after the first frame anyway, so basically it
didn't work)

Columns: tidying up, moving OffsetT to a ImGuiColumnData structure

Rename internal ColumnsStartX to IndentX, makes more sense

InputText: Added password mode via ImGuiInputTextFlags_Password (#237, #363, #374)

ImageButton() uses FrameRounding up to a maximum of available framing size (#394)

Minor popups comment tweak (#402)