The Dialog Refactoring

[sppmacd]

This mainly adds WindowRoot class, that represents widget hierarchy + window attributes in a single object. This is mainly to support loading dialogs from EML (see e.g FileExplorer) while allowing them to be any kind of window (HostWindow, MDI::Window). Also, in future I want to make some "tiling window manager" (e.g for EssaDB to support resizable splitters, docking etc).

• Port everything to WindowRoot ("Screen") API:
  • FilePrompt
  • MessageBox
  • Prompt
  • ColorPickerDialog
• Don't use legacy MDI hack in examples and remove it
• Port special windows to native (Host) windows:
  • ContextMenuOverlay (uses widgets, so should be easy) turned out to be not so easy because of totally wrong event logic in modal loop
  • TooltipOverlay (maybe just a label)
  • NotificationWindow (??? doesn't realliy fit anywhere, maybe just should use BasicLayout & proper z-index support)
• Merge MDI::Overlay+MDI::Window and make it final
• Rename WidgetTreeRoot -> something ?? (AbstractWindow)
• Remove MDI::Window indirection in MDI::Host
• Add and use async Dialog class (required for full Emscripten support)
• Don't support modal MDI windows (this doesn't make sense for random MDI widget to block the entire program ??)
• ...