Brought back LiveEditorApp which now uses TextFileWidget and ProgramWidget.

If ProgramWidget has a target on construction, then perform an OnChange.
Removed Project::GetSourceOnChange(), moved it to LiveEditorApp.
TextFileWidget gains its own m_OnChange.
ConnectionWidget and ProgramWidget's target can be changed externally
using code via SetTarget().
CanvasWidget's background now supports non-centered mode too.
FlowLayoutWidget's Render() respects m_Visible.