Some of the toolbar buttons are seriously wrong when mirrored and there
doesn't seem to be a way to disable mirroring for the images while using
RTL layout for the toolbar itself, so double-mirror them so that they
end up with their original appearance.
Ideally the button images would be localizable as some of them actually
should be mirrored, but that's probably not worth the work involved.
See #1354.
istringstream makes a copy of its input, which is not very nice for
performance. strstream doesn't do bounds checking, which is not very
nice for safety (and is deprecated).
