qmlls: watch C++ files and trigger qrc, qmltypes, qmldir generation

Use https://doc.qt.io/qt-6/qfilesystemwatcher.html to watch the C++ files containing QML components: if they are changed, trigger the generation of qrc, qmltypes and qmldir files via cmake.

The C++ files containing components can be found via the qmltypes: each component there contains the path to the header defining it.