Make QML compilation optional

Precompiling QML code makes ktouch hard-depend on the Qt version it is compiled with, forcing it to be rebuilt with every new Qt version. This creates an additional burden for packagers in exchange for a negligible performance gain the first time the application is run (for every subsequent run the QML code is already compiled in the cache anyway)

Differential Revision: https://phabricator.kde.org/D20861