Try hard to keep the currently selected key/item if the model changes

This ensures that the selection does not change for example after
setting the owner trust of the currently selected key.

* Store the current selection if an imminent model change is announced
* Restore (if possible) the selection after the model change
* Tell the model to use the key cache and remove the custom "Loading
  keys" item only after the initial key listing.
* Select the default key only after the initial key listing.
* In setCurrentKey() look up the key by fingerprint instead of by the
  key object because the latter fails if the key was updated.
* In setCurrentKey() fall back to selecting the default key if the
  given key isn't found and selectPerfectIdMatch() also fails.
* Add removeCustomItem() (currently only used in the test).

GnuPG-bug-id: 5511