Unbreak Qt6 build

@vkrause @nicolasfella @davidedmundson @apol

To get around the hurdle as mentioned in !37 (merged), this patch introduces an option to build support for the global DBus menu, named BUILD_DBUSGLOBALMENU_SUPPORT, default ON for backward-consistency.

To get CI starting to dover the Qt6 build, the CI config is extended to build for Qt6 without the support. For developers though the default stays ON, so when building with Qt6 they keep running (at least initially) into this open TODO.

While this is a work-around to get things building for Qt6, is this option perhaps useful for other deployments of plasma-integration, so it makes more sense to add this?