reorganization: Rename extensions -> language

I want all language elements to be defined in the same folder, rather
than having the "core" language split by parsing/AST/completions and the rest
split by language element.

blueprintcompiler/language/__init__.py

@@ -0,0 +1,19 @@
+""" Contains all the syntax beyond basic objects, properties, signal, and
+templates. """
+
+from .gobject_signal import Signal
+from .gtk_a11y import A11y
+from .gtk_combo_box_text import Items
+from .gtk_file_filter import mime_types, patterns, suffixes
+from .gtk_layout import Layout
+from .gtk_menu import menu
+from .gtk_size_group import Widgets
+from .gtk_string_list import Strings
+from .gtk_styles import Styles
+
+OBJECT_HOOKS = [menu]
+
+OBJECT_CONTENT_HOOKS = [
+    Signal, A11y, Styles, Layout, mime_types, patterns, suffixes, Widgets, Items,
+    Strings,
+]