Tabs to spaces

Added GetScrollX(), GetScrollX(), GetScrollMaxX() apis + demo (#246)

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

Fixed initial ScrollbarX decision when no vertical scrollbar is in (#246)

BeginChild() fixed missing rounding for child sizes which leaked into layout

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

TextUnformatted(): simplifying

Separator(): removed unnecessary computation + comment.

ColumnsStartX include window->Scroll.x to simplify code (#246)

Style: minor menubar bg alpha tweak.

Style: made scrollbars rounded by default. Because nice. (#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.

Bullet() fixed inconsistent layout behaviour when clipped.

Added GetContentRegionAvailWidth() helper. Demo tweaks.

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

Tidying up

Added GetCurrentWindowRead() for functions that don't write to the window

Fixed OpenPopup() accessing current window, effectively opening "Debug" on an empty window stack

Minor internal shortcuts not calling GetCurrentWindow()

Style: rename ScrollbarWidth to ScrollbarSize (sorry!)

TreeNode() fixed IsItemHovered() being inconsistent with visual behavior (#282)

Split tree/collapsable into internal TreeNodeBehaviorIsOpened() helper (unsure..) (#282)

TreeNode(): arbitrary interaction padding after text label not part of layout (#282)

IO.WantInputCharacters -> WantTextInput (#305)

InvisibleButton() honor negative size consistently with other widgets that do so

ImDrawList: added AddBezier(), PathBezierTo() for cubic bezier curves (#311)

Examples: OpenGL: fix for retina screens + io.DisplayFramebufferScale storage (#287)

Malformed UTF-8 don't terminate string, output 0xFFFD (#307)

Added ColorConvertU32ToFloat4() helper and ImColor(ImU32) constructor.

Added GetWindowHeight() for completeness + BeginGroup() comment.