JuK optimizapalooza!

* Add whatever X-DBUS-foo line KUniqueApplication says we need.
* Restructure the startup sequence to postpone all the hard work until after the GUI is setup as much as possible to allow passing our reply over DBUS ASAP.
* Don't call hasCover from PlaylistItem like, ever. I made CoverInfo::hasCover() take longer when I added support for APIC frames but didn't catch all unnecessary instances of their use last time. Luckily this is the big win and is backportable.
* Also, don't force CoverInfo::coverId() to call hasCover() for no reason.

Hopefully this should remove the egregrious Plasma freeze-ups while starting but even after returning from KUniqueApplication::newInstance() quickly it seems to take a while for the reply to go over the D-BUS...