icontheme: Optimize memory use and lookup speed by internalizing icon names

icontheme: Optimize memory use and lookup speed by internalizing icon names

Instead of having the IconTheme have a hashtable that owns
individual strings and then IconThemeDirSize have a similar
hash (but with the strings owned by the other hash), we
have a consecutive memory chunks where we store the icon names
and then the hashtable has pointers into this.

This means we can avoid a bunch of individual strdup()s in a
way that is less fragmented and wastes less space. Additionally,
since we do an initial lookup anyway we have the internalized
icon name during lookup which means we can use g_direct_hash/equal
instead of g_str_hash/equal making the lookup faster too.