[core/theme] Add support for icon themes

Allow sub-themes ("iconsets") to be merged into the "main" theme. That
way, effectively, it's possible to define colors and icons in separate
JSON files.

see #23

tests/util.py

@@ -22,10 +22,17 @@ class MockOutput(object):
 class MockWidget(object):
     def __init__(self, text):
         self._text = text
+        self._module = None
+
+    def set_module(self, name):
+        self._module = name
 
     def update(self, widgets):
         pass
 
+    def module(self):
+        return self._module
+
     def full_text(self):
         return self._text