GDesktopAppInfo: fix default app logic

We use "tweaks" structures to track how a particular directory impacts
the list of added, removed and default applications.  We maintain this
set of tweaks for each directory, in a hash table, keyed by unaliased
mime type name, in order to facilitate fast lookups.

A typo in the logic for creating and maintaining the uniqueness of these
structures was causing the default app to be selected incorrectly from
time to time.  Fix that.