[core/theme] Add preparatory work for themes

Add two new parameters: theme and iconset

Add a placeholder class core.theme.Theme, an instance of which is passed
in to the i3 output object (which is the only object that should ever
have need of the theme, hopefully).

core/output.py

@@ -2,8 +2,10 @@ import sys
 import json
 import time
 
+import core.theme
+
 class i3(object):
-    def __init__(self):
+    def __init__(self, theme=core.theme.Theme()):
         self._modules = []
         self._status = {}