[core/theme] Add iconset support

Allow themes to specify iconsets. To do so, add a new util library
"util.algorithm", which currently contains support for deep merging of
dicts.

util/algorithm.py

@@ -0,0 +1,23 @@
+import copy
+
+# algorithm copied from
+# http://blog.impressiver.com/post/31434674390/deep-merge-multiple-python-dicts
+# nicely done :)
+def merge(target, *args):
+    if len(args) > 1:
+        for item in args:
+            merge(target, item)
+        return target
+
+    item = args[0]
+    if not isinstance(item, dict):
+        return item
+    for key, value in item.items():
+        if key in target and isinstance(target[key], dict):
+            merge(target[key], value)
+        else:
+            if not key in target:
+                target[key] = copy.deepcopy(value)
+    return target
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4