[core] Use class theme, if no specific module theme matches

For modules that use aliases, until now, *only* the alias theme was
considered. This is typically a bad idea (tm), as the "generic" case is
that of a module theme (e.g. "disk", not that of a theme per instance of
the module (i.e. "home", "root", etc.)).

Therefore, introduce the concept of a "class" theme that can be
optionall overridden by a specific module theme.

As a pleasant side-effect, this should bring back the disk icons for all
instances of the disk module :)

fixes #79

bumblebee/output.py

@@ -47,6 +47,11 @@ class Widget(bumblebee.store.Store):
         self.module = module.name
         self._module = module
 
+    def cls(self):
+        if not self._module:
+            return None
+        return self._module.__module__.replace("bumblebee.modules.", "")
+
     def state(self):
         """Return the widget's state"""
         if self._module and hasattr(self._module, "state"):

bumblebee/theme.py

@@ -130,6 +130,7 @@ class Theme(object):
                 self._cycle = self._cycles[self._cycle_idx]
 
         module_theme = self._theme.get(widget.module, {})
+        class_theme = self._theme.get(widget.cls(), {})
 
         state_themes = []
         # avoid infinite recursion
@@ -141,6 +142,7 @@ class Theme(object):
         value = self._defaults.get(name, default)
         value = widget.get("theme.{}".format(name), value)
         value = self._cycle.get(name, value)
+        value = class_theme.get(name, value)
         value = module_theme.get(name, value)
 
         for theme in state_themes: