load geokrety in GC cache inventory

This is for issue #3007. Now we can see Geokrety in the GC inventory.
Hope this help...

* rework xml parsing a bit
* force loading Geokrety when refreshing GC cache
* add LoadProgressDetail string for inventory loading

(This is my first android app contribution. Please be indulgent)
I've failed implementing Loging GK, due to android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork exception.
All network access should use the methods provided in the `Network` package. This will help the migration towards a non-blocking architecture based on observables later.