Specify Dependency on XmlGui 5.1

Needed since f92a534c8d774f09e91941d2c3ae4648e033bb55

REVIEW: 119661
add a .kateproject file
Fix plugin unloading for custom-definesandincludes

This wasted 30 mins of my life, because I've totally overlooked the
'static' while debugging the dangling pointer access :/

Funnily, this only showed up for the bazaar/git tests, which actually
restart the Core for every test *method*.
Port to kdevplatform_add_test