Make Krita a QtSingleApplication

CCMAIL:kimageshop@kde.org

If there is a running instance of Krita, and you open another image with Krita,
or start Krita from the commandline, the files in the arguments are opened in
the first Krita.

This also solves synchronization issues with the tag store, as well as possible
issues with setting shortcuts. (Though not all shortcut issues are solved yet).

BUG:342187 Possibility to open images in one krita app
BUG:318208 With multiple instance of Krita open, preset tags get out of sync