Add explicit moc includes to cpp files, simplify how generated UI classes are handled

The rationale is explained in https://planet.kde.org/friedrich-kossebau-2023-06-28-include-also-moc-files-of-headers/

In case of KDEConnect, it impressively speeds up compilation. Before it took 390 seconds on a clean build and with this change it took 330 seconds. This is due to the mocs_compilation having to include the header files and thus all their headers. Due to the lots of small plugins we have, this means that the same headers must be compiled plenty of times. When we include the moc files directly in the C++ file, they are already available.