Split FileTree.{h,cc} and FilterBar.{h,cc} files so that each class
is in its own file.
This breaks translations (some classes got renamed => context changed),
to be fixed by next commit (along with Tx sync).
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
Mike Gelfand