Image widget fixes and scroll improvements

Added

• Support for switching vertical wheel scrolling to horizontal in scroll widget by pressing the shift key (PR #137).
• Drawing and theme utility functions (PR #138).
• boxFilterEvent config option, exposing Container's filterEvent functionality (PR #146).
• pointToLineDistance utility function (PR #150).

Fixed

• Bug where memory based image widget would not render their new state after a merge (PR #147). Thanks @CamdenKuwahara!
• Fix memory leak in NanoVGRenderer's image initialization. Dispose unused images in image widget (PR #149).

Changed

• Restricts toggleButton's type argument for configuration options to Bool (the only valid type) (PR #150).
• Avoids drawing borders if width == 0 (PR #150).
• Updates default style for tooltip widget (PR #150).