config: support <mouse><default />

config: support <mouse><default />

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 <default />
element is not provided.

Example usage:

  <mouse>
    <default />
    <context name="Root">
      <mousebind button="Right" action="Press">
        <action name="Execute" command="my-menu-application" />
      </mousebind>
    </context>
  </mouse>