This adds an optional <unique-id> element to the <action> element in
uca.xml. When loading the UCA model from uca.xml, we check for this
element in the parser. If it is not given, we generate a new ID with the
following signature:

  <current_time.tv_sec>.<current_time.tv_usec>-<static counter value>

After parsing we check if we have generated new IDs and, if so,
immediately save the model back to the uca.xml file. This part is a bit
of a hack but it will be a no-brainer once we've switched to xfconf, I
think.

The ThunarUcaEditor is extended so that it generates a unique ID for new
items with the same signature as above. It does not overwrite IDs of
existing custom actions.

Together this allows for a persistent association of custom actions with
keyboard shortcuts.