qmlls: automatic qmltypes generation

Currently, the user needs to build its QML modules for qmlls to provide any helpful insights. This is bad because a new qmlls user that uses it for the first time will have a very bad first impression of qmlls, as it will not be able to do anything.

Also, currently, if the user changes its QML component defined in C++, the qmlls will not know about any added/removed/changed property or new classes.

Therefore, it was proposed that qmlls should be able to trigger the rebuild of the "autogen" (that generates the qmltypes, qmldir and qrc files) so that the user always get up-to-date linting warnings and completions items.

The "easy" way of doing it would be to:

• create a cmake target that can regenerate the qmltypes, qmldir and qrc files (that is faster than rebuilding the entire project of the user)
• Trigger this cmake target from qmlls automatically/without the user needing to do it:
  • when no qmldir nor qmltypes was found (e.g. when opening a newly created QML project that was never built yet)
  • when a C++ file defining QML components was changed (e.g. a property was added/changed/removed)