Don't load all debugging symbols by default when debugging QML/CPP app

When a user now debugs any cpp app, all the debugging symbols are loaded, including Qt and all other kinds of stuff. This can be very slow and makes the user want to only debug in QML mode (which is very fast).

To facilitate this, the user should be able to select which symbols to load (default: all, as now), so that if Qt symbols are not needed, only their own app/libs symbols would be loaded.

This could also be a generic debugger feature, but for QML-related stuff, it would be quite useful.