DesktopStreamer: Improvement for OSX application streaming

[dnachbaur]

From #61:

• Hidden windows (at the time of launching DesktopStreamer) never appear in the list, even after made visible. (addressed in DesktopStreamer: update list after (un)hiding windows #97)
• Fullscreen applications (i.e those on their own "Desktop") are not shown in the list
• Only the main virtual desktop can be streamed
• The list of windows is perfectly functional but not yet super nice to navigate, maybe we can try to improve the look and feel later
• If window hidden (or image size() is 0), don't stream anymore. (addressed in DesktopStreamer: update list after (un)hiding windows #97)
• Show mouse cursor only on window below the mouse cursor
• Added/removed window(s) for an already open app are not shown in the list
• Apps with more than one window don't stream any image (use case: two X11 windows from vglrun)
• Hiding, or closing, one of multiple windows of an application leaves a window behind in the list. Closing the full application does not even make it disappear.
• Clicking in an application window brings it fullscreen, but the click event itself may be lost if the operation is not fast enough.
• Use deflect::SizeHints to support resize of streamed window(s)
• Download pixmap in background thread[s] for better performance
• Only download changed windows
• Change the UI so it's more intuitive. It's not obvious that there is a 'Stream' button that needs to be clicked after selecting the windows