So my questions are:
* How do I find out from the button-release-event, whether the icon was
double-clicked?
There is no builtin support for double-clicks, so you'll have to implement it yourself:
You need to record the time and location of the last release event, and then check whether the next event is close enough to be considered a double-click. Use either ClutterSettings:double-click-time/ClutterSettings:double-click-distance or GtkSettings:gtk-double-click-time/GtkSettings:double-click-distance for the thresholds to be consistent with other components.
* Is there a way to check library versions from within lg/gjs?
Not generically. If a library defines a version symbol like CLUTTER_VERSION, it will be available to introspection (Clutter.VERSION) though.
* Why is get_event_type not available? Or am I looking at the wrong
get_button()? Where is the correct one documented?
gnome-shell is based on Clutter rather than Gdk/Gtk+, so you are indeed looking at the wrong get_button() - you want ClutterEvent instead of GdkEvent. Both APIs are very similar, but the method to get the event type is called event_type() in Clutter.
