[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

bumblebee/theme.py

@@ -1,6 +1,7 @@
 """Theme support"""
 
 import os
+import copy
 import json
 
 import bumblebee.error
@@ -12,11 +13,13 @@ def theme_path():
 class Theme(object):
     """Represents a collection of icons and colors"""
     def __init__(self, name):
-        theme = self.load(name)
         self._init(self.load(name))
 
     def _init(self, data):
         """Initialize theme from data structure"""
+        for iconset in data.get("icons", []):
+            self._merge(data, self._load_icons(iconset))
+        self._theme = data
         self._defaults = data.get("defaults", {})
 
     def prefix(self, widget):
@@ -31,10 +34,14 @@ class Theme(object):
         theme = json.loads(data)
         self._init(theme)
 
-    def load(self, name):
+    def _load_icons(self, name):
+        path = "{}/icons/".format(theme_path())
+        return self.load(name, path=path)
+
+    def load(self, name, path=theme_path()):
         """Load and parse a theme file"""
-        path = theme_path()
         themefile = "{}/{}.json".format(path, name)
+
         if os.path.isfile(themefile):
             try:
                 with open(themefile) as data:
@@ -44,10 +51,32 @@ class Theme(object):
         else:
             raise bumblebee.error.ThemeLoadError("no such theme: {}".format(name))
 
-    def _get(self, widget, name,default=None):
-        value = default
-        value = self._defaults.get(name, value)
+    def _get(self, widget, name, default=None):
+
+        module_theme = self._theme.get(widget.module(), {})
+
+        value = self._defaults.get(name, default)
+        value = module_theme.get(name, value)
 
         return value
 
+    # algorithm copied from
+    # http://blog.impressiver.com/post/31434674390/deep-merge-multiple-python-dicts
+    # nicely done :)
+    def _merge(self, target, *args):
+        if len(args) > 1:
+            for item in args:
+                self._merge(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):
+                self._merge(target[key], value)
+            else:
+                target[key] = copy.deepcopy(value)
+        return target
+
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