Xpra: Ticket #539: tabbed client

Instead of displaying all the client windows as individual windows, we could use a tabbed viewer which shows each client window as a separate tab.

Problems:

• Application Quircks: applications like google chrome that don't use the correct window type hints will just be impossible to handle correctly
• refactoring needed for:
  • client window base vs client widget base common code
  • clicks, mouse, keyboard handling, focus, etc
  • painting spinners
  • opengl backing integration (potentially big, could go with a simple pixmap backing to begin with: lots of memory copying, but client usually has CPU to spare anyway)
• pass icons and title to tab parent window
• missing code for: xid
• decide how to handle: applications that request a resize or move
• potentially very big: smooth scrolling (allow the server side window to be much bigger than what we display and use scrollbars to navigate it) - could be extended later so that the remote app cooperates in scrolling (so we can have a virtual window size much bigger than the server-side window area)
• handle maximize, fullscreen
• slow down the refresh for the tabs that aren't shown (but still update our back buffer so we can show them instantly when switching tabs)
• handling size hints, etc: we need to add the offset of our top bar (should be possible by modifying the base-size attribute on the fly?)

I will attach a stub client that uses virtual "tab" windows (work in progress)