Open With: don't raise KeyError if cache exists and there are new dirs

If the cache failed to load, it is initialized as a defaultdict and all
mtimes compare as 0. If the cache did load, however, then an ordinary
dict was used, and if new icon directories appeared on the system since
the cache creation, they would raise a KeyError and Open With would not
load data.

Fix by using a defaultdict in all cases, but initializing with the
contents of the cache if possible.

Discovered when crazy applications added crazy subdirectories in
/usr/share/pixmaps (???) and suddenly calibre failed to do the right
thing, but the same should apply if the system adds a new icon theme.

src/calibre/utils/open_with/linux.py

@@ -116,7 +116,7 @@ def find_icons():
         with open(cache_file, 'rb') as f:
             cache = f.read()
         cache = msgpack_loads(cache)
-        mtimes, cache = cache['mtimes'], cache['data']
+        mtimes, cache = defaultdict(int, cache['mtimes']), defaultdict(dict, cache['data'])
     except Exception:
         mtimes, cache = defaultdict(int), defaultdict(dict)