rcxml: convert dotted properties into nested nodes before processing

For example, the following node:

  <keybind name.action="ShowMenu" menu.action="root-menu"
            x.position.action="1" y.position.action="2" />

is converted to:

 <keybind>
   <action>
     <name>ShowMenu</name>
     <menu>root-menu</menu>
     <position>
       <x>1</x>
       <y>2</y>
     </position>
   </action>
 </keybind>

...before processing the entire xml tree. This is a preparation to prevent
breaking changes when we refactor rcxml.c to use recursion instead of
encoding nodes into dotted strings.

include/common/xml.h

@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_XML_H
+#define LABWC_XML_H
+
+#include <libxml/tree.h>
+
+/*
+ * Converts dotted attributes into nested nodes.
+ * For example, the following node:
+ *
+ *   <keybind name.action="ShowMenu" menu.action="root-menu"
+ *            x.position.action="1" y.position.action="2" />
+ *
+ * is converted to:
+ *
+ *   <keybind>
+ *     <action>
+ *       <name>ShowMenu</name>
+ *       <menu>root-menu</menu>
+ *       <position>
+ *         <x>1</x>
+ *         <y>2</y>
+ *       </position>
+ *     </action>
+ *   </keybind>
+ */
+void lab_xml_expand_dotted_attributes(xmlNode *root);
+
+#endif /* LABWC_XML_H */