Reorganize the parser/AST code

The code is now organized by syntax: `menu {}` in one file, `style` in
another, etc. This should make it easier to add syntax in the future.

gtkblueprinttool/extensions/__init__.py

@@ -0,0 +1,10 @@
+""" Contains all the syntax beyond basic objects, properties, signal, and
+templates. """
+
+from .gtk_menu import menu
+from .gtk_styles import styles
+from .gtk_layout import layout
+
+OBJECT_HOOKS = [menu]
+
+OBJECT_CONTENT_HOOKS = [styles, layout]