We don't currently (if ever) provide context help, so the button is
useless. Moreover, on Windows 10 it's even larger than before and
sometimes title text doesn't fit because of it.
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).