qmake: replace custom system with a boring qmake setup

Antti Poikela <antti.poikela@iki.fi> (amppa on IRC) reported that trojita failed
to link with mimetypes-qt4 on Windows. This is due to the path manipulation
which Trojita's own linking.pri performs on Windows. That path manipulation is
in turn required because of qmake not tracking dependencies on static libraries
by default (at least that's how it worked back in 2009). Let's just throw away
the complex and non-standard setup of mimetypes-qt4 with something which might
have a chance of working on Windows.

Also complain loudly when the QStandardPaths emulation is not available --
that's the best we can do instead of failing later during build.
GUI: add some splitters to the composer widget

refs #578
SVN_SILENT made messages (.desktop file)
GUI: don't show a "Multipart Message" indicator in the GUI

Some people (iirc serpentinbe on IRC) argued that this does not make much sense.
I guess I can agree with them.
GUI: make it clearer that this is for showing another message-in-a-message
qmake: fix missing library on Mac

Thanks to Markus Goetz (guruz on IRC) for reporting. Looks like I was too eager
when nuking the qmake nasties in 9dced46dad2674caabadfeb6cbf43aa082fbcea4.
Fix finding `lrelease` on OpenBSD

On OpenBSD and maybe other platforms lconvert is installed in a non-default
location so it's not found during build. The attached patch mimics the process
of setting QMAKE_LRELEASE for lconvert.

fixes #618