Begin() windows flags are locked on the first begin of the frame, allowing using to not bother passing flags for append

GetContentRegionMax() when window content size is explicit we don't adjust it with scrollbar sizes (#246)

Added optional Horizontal Scrollbar & allow user to explicitly set content width via SetNextWindowContentSize() etc (#246)

GetCursorPos, SetCursorPos, GetContentRegionMax,
GetWindowContentRegionMin, GetWindowContentRegionMax are now including
the scrolling amount. It PROBABLY shouldn't break anything for you, but
take note that SetCursorPosX(100.0f) puts you at +100 from the starting
x position which may include scrolling, not at +100 from the window left
side.

Turning float ScrollY into ImVec2 Scroll - no feature change, this is just to remove some syntax noise from the following commit (#246)

Might save an hour of staring at blank-screen to 5% of the population,
worth it.