[core] Collapse modules by using middle mouse

When pressing the middle mouse button (and it's not assigned to any
other functionality), the module (i.e. all widgets of that module) will
disappear and be replaced with the module's icon (or prefix, as
fallback) and an ellipsis.

fixes #264

bumblebee/output.py

@@ -102,10 +102,12 @@ class I3BarOutput(object):
         padding = self._theme.padding(widget)
         prefix = self._theme.prefix(widget, padding)
         suffix = self._theme.suffix(widget, padding)
+
         if prefix:
             full_text = u"{}{}".format(prefix, full_text)
         if suffix:
             full_text = u"{}{}".format(full_text, suffix)
+
         separator = self._theme.separator(widget)
         if separator:
             self._widgets.append({