Doc: rewrite Translators topic in Qt Linguist Manual

Make the topic task-oriented. Move information around and
remove redundancy. Check the paths to commands and availability
of commands. Make docs reflect some changes in functionality.
Remove images of toobar buttons that are no longer referred to
in text.

Change-Id: I7f9743d35ceebfc0f7097a1af2e12eeb344c8e58
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>