Create very basic message tree widget structure

Also, fix some cheuph structuring problems

cheuph/rendered_message_cache.py

@@ -0,0 +1,10 @@
+__all__ = ["RenderedMessageCache"]
+
+
+class RenderedMessageCache:
+    """
+    This is a cache for RenderedMessage-s. Message-s should not need to be
+    redrawn every frame (and every in-between calculation).
+    """
+
+    pass