[core] Widget creation/update overhaul

Until now, widgets were re-created during each iteration. For multiple,
reasons, using static widget objects is much easier, so instead of
creating new widgets continuously, modules now create the widgets during
instantiation and get the list of widgets passed as parameter whenever
an update occurs. During the update, they can still manipulate the
widget list by removing and adding elements as needed.

Advantages:
* Less memory fragmentation (fewer (de)allocations)
* Easier event management (widgets now have static IDs)
* Easier module code (widget contents can simply be the result of a
  callback)

see #23

bumblebee/modules/test.py

@@ -6,9 +6,11 @@ import bumblebee.engine
 
 class Module(bumblebee.engine.Module):
     def __init__(self, engine):
-        super(Module, self).__init__(engine)
+        super(Module, self).__init__(engine,
+            bumblebee.output.Widget(full_text="test")
+        )
 
-    def widgets(self):
-        return bumblebee.output.Widget(full_text="test")
+    def update(self, widgets):
+        pass
 
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