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.

src/common/meson.build

@@ -23,4 +23,5 @@ labwc_sources += files(
   'surface-helpers.c',
   'spawn.c',
   'string-helpers.c',
+  'xml.c',
 )