rcxml: fix broken titlebar layout with multiple <layout> entries

rcxml: fix broken titlebar layout with multiple <layout> entries

Before this commit, when we have multiple <theme><titlebar><layout>
entries like below, duplicated button types can be inserted to
rc.title_buttons_{left,right} and the button could go outside of the
window:
    <theme>
      <titlebar><layout>icon:iconify,max,close</layout></titlebar>
      <titlebar><layout>icon:iconify,max,close</layout></titlebar>
    </theme>

This commit fixes by clearing those lists when the parser encounters
<theme><titlebar><layout>.