qmlls: concurrent qmltypes generation

Investigate if the mechanism introduced by https://codereview.qt-project.org/c/qt/qtdeclarative/+/518320/3 can be made concurrent.

One naive approach could be to create the Dom while, in a separate thread, CMake is rebuilding the qmltypes, qmldir and qrc files. Once the CMake is finished, recreate the Dom and replace the previoulsy created Dom on outdated information.