Fix Qt resource handling

Turns out we have to explicitly initialize Qt resources that are linked
statically, so our last build system change broke things because the
files where nowhere to be found.

This now initializes the resources in main(); additionally, we use absolute
paths for the qrc files now, simply for making things a bit clearer.
Bring back COMMON_DEPS

This is actually being used to link the app icon on Windows, so bring
the old variables back for now. Probably want to clean this up later
though, as the naming is confusing.