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/gtk_layout.py

@@ -0,0 +1,56 @@
+# gtk_layout.py
+#
+# Copyright 2021 James Westman <james@jwestman.net>
+#
+# This file is free software; you can redistribute it and/or modify it
+# under the terms of the GNU Lesser General Public License as
+# published by the Free Software Foundation; either version 3 of the
+# License, or (at your option) any later version.
+#
+# This file is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# SPDX-License-Identifier: LGPL-3.0-or-later
+
+
+from ..ast_utils import AstNode, BaseAttribute
+from ..completions_utils import *
+from ..parse_tree import *
+from ..parser_utils import *
+from ..xml_emitter import XmlEmitter
+
+
+class Layout(AstNode):
+    def emit_xml(self, xml: XmlEmitter):
+        xml.start_tag("layout")
+        for child in self.children:
+            child.emit_xml(xml)
+        xml.end_tag()
+
+
+class LayoutProperty(BaseAttribute):
+    tag_name = "property"
+
+
+layout_prop = Group(
+    LayoutProperty,
+    Statement(
+        UseIdent("name"),
+        Op(":"),
+        value.expected("a value"),
+    )
+)
+
+layout = Group(
+    Layout,
+    Sequence(
+        Keyword("layout"),
+        OpenBlock().expected("`{`"),
+        Until(layout_prop, CloseBlock()),
+    )
+)