MIDebuggerPlugin: setup actions only if there is a QGuiApplication
((stump branch in place of old commit)
No better idea what to do here. Using qApp over the old code, to be consistent with similar checks across the codebase.
test_midbus is a UI-less test, In Qt6 the creation of QAction instances tries to use a QGuiApplication instance and will crash hard if there is none.
No access to Core::setupFlags in the plugins, only to ICore, so other than e.g. DebugController::initializeUi() MIDebuggerPlugin cannot check that. Thus using the same approach as other "are-we-ui" non-kdevelop-core code.