GUI: add some splitters to the composer widget

refs #578
SVN_SILENT made messages (.desktop file)
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.
Merge branch 'recipients-splitters'
Merge remote-tracking branch 'kde/master'
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
Update copyright details for new contributors
website: point to relative URLs instead of absolute URLs

This makes it possible to browse the website from the Git repository, for example.
cleanup: remove unused variable
Mac: Fix compile

This builds upon an incomplete fix in e954cc11e45667409dbc0c654596cff1adcd8010
which is not enough due to the way how the qmake dependencies on static libs
work.

Signed-off-by: Jan Kundrát <jkt@flaska.net>
Mark the files read-only when saving attachments for direct-open

This was suggested by Thomas Kluyver <thomas@kluyver.me.uk> on the xdg mailing
list.
Fix broken templates in the unit tests

At first I thought these were legit, but in fact this has always relied on
linker being "smarter" than required.

Reported by Antti Poikela <antti.poikela@iki.fi> (amppa on IRC) on, well, IRC.
Gui: proper menu items for editting of draft messages

Thanks to Ronny Pfannschmidt <Ronny.Pfannschmidt@gmx.de> (ronny on IRC) for
reporting that this is not available from the menu.
Gui: better icons and strings for the composer actions
Merge branch 'master', remote-tracking branch 'upstream/master'