Shaperilio/qtgui fixes (#13957) I started using the released version of my `PySide6`-enabling changes and noted some problems. In this PR, I fix those, and also overall improve the feedback to the user when a GUI event loop is hooked in: - Report which event loop is running when using `%gui <some GUI>`; e.g. `%gui qt` will show `Installed qt6 event loop hook.` - Report when the event loop is disabled; i.e. `%gui` will show `GUI event loop hook disabled.` if an event loop hook was installed, or `No event loop hook running.` if nothing was installed. - Requesting a second event loop will give the message `Shell is already running a gui event loop for <some GUI>. Call with no arguments to disable current loop.` - Requesting a different version of Qt, i.e. `%gui qt6` followed by `%gui` followed by `%gui qt5` will show `Cannot switch Qt versions for this session; will use qt6.` followed by `Installed qt6 event loop hook.` (Fixes / improves #13864)