Add g_desktop_app_info_search()

The first time this function is called we load all of the keyfiles in
the directory, ignoring the 'Hidden' ones and build an index out of the
interesting fields using g_str_tokenize_and_fold().

We do prefix matching on the tokens to find relevent desktop files.

Right now this is implemented as a hashtable that we iterate over,
checking prefixes on each token.  This could possibly be sped up by
creating an array, but it's already pretty fast...

https://bugzilla.gnome.org/show_bug.cgi?id=711557