LSP: find usages in qml

Find usages should be provided, as it is the basis of several refactoring operations.

We can distinguish local symbols that are defined in the js code in qml, and the "exported/imported" symbols that can be seen by all files.

Building the list of exported symbols means scanning all files in a project.

It might be useful to use a SQLLite approach for this, to avoid having to rescan everything if not needed.

Work by Marco Bubke might be useful for this.