Undo IsHovered > IsItemHovered, shorter name wins

Tweaks, more consistent #define names

Added GetItemBoxMin(),GetItemBoxMax(), renamed IsHovered()-->IsItemHovered()

Added InputFloat2(), SliderFloat2()

Fix for gcc type qualifier warnings. ...

With -Wall -Wextra -Werror, it is not possible to compile against
imgui.h due to const correctness violation in ImVector.

Simplified ImDrawList system (samples are 20 lines shorter) + merged title bar draw bar.

Fixed warnings for more stringent compilation settings. Added various small helpers.

Missing assert + comments

Tweaked checkbox/radio default hover color to be less distracting. Added GetItemWidth(). Added text filter to style editor.

Added ImGuiCol_CheckHovered to show hovered checkboxes and radio buttons

Commented samples and shuffled bits of the initialisation based on user's feedback.

Added PixelCenterOffset for OpenGL/DirectX compatibility.

Initial release