Move status bar creation to mainwin.ui

Among other things,
* fix turtle icons size (20x14 -> 16x16)
* use QIcon states instead of changing the turtle icon ourselves each
  time (unchecked - off, checked - on)
* make speed limit action in tray menu checkable
* simplify status bar buttons drawing and use QToolButton instead of
  QPushButton


git-svn-id: svn://svn.transmissionbt.com/Transmission/trunk@14389 f4695dd4-2c0a-0410-b89c-da849a56a58e