Fix handling %U when launching multiple files with an app (bug #7456).

GIO returns a newly allocated GAppInfo every time
g_app_info_get_default_for_type() is called. This means that if we use a
GHashTable and g_direct_hash() to associate GAppInfos with files to be
launched with each of them, we will actually end up with multiple
GAppInfos that are the same, and each of them will only have a single
file associated.

To fix this, we now use a fake hash function that causes GHashTable to
always search the GAppInfo in the collision list.