Changelog:
- New backend: raylib
- New widget: EditBoxSlider (PR #238)
- All widgets can now be configured to ignore mouse events
- Added HorizontalLayout and VerticalLayout to replace widget-specific enums
- Added method to associate user data to combo box items
- Added onWindowFocus and onWindowUnfocus signals
- Renamed isKeptInParent to getKeepInParent in ChildWindow
- Renamed limitTextWidth to setTextWidthLimited in EditBox
- String::fromNumber now supports int8_t (but no longer accepts pointers)
- BackendTextureSFML::getInternalTexture() now returns a pointer
- BackendFontSFML::getInternalFont() now return a pointer