src/config/rcxml.c: Add support for <devault /> mousebinds

src/config/rcxml.c: Add support for <devault /> mousebinds

This loads default mousebinds and provides a way to keep config files
simpler whilst allowing user specific binds.

Note that if no rc.xml is found, or if no <mouse><mousebind> entries
exist, the same default mousebinds will be loaded even if the <devault />
element is not provided.

Example usage (with a slight spelling error):

  <mouse>
    <default />
    <context name="Root">
      <mousebind button="Right" action="Press">
        <action name="ShowMenu" menu="desktop-menu" />
      </mousebind>
    </context>
  </mouse>

Co-Authored-By: @johanmalm
Fixes #416