This has a couple of benefits: 1) it is clearly visible to the user that
the list is filtered (doesn't display all the torrents) even when filter
bar is hidden, 2) doesn't lead to filter bar controls being shifted to
the left/right as when "Show:" label text changes.
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).
