window-rules: support matchOnce attribute

window-rules: support matchOnce attribute

...allowing a rule to be applied to only the first window matching a
particular criteria. For example, the following can be used to apply a
window rule to lxqt-panel but not its configuration window with the same
app_id:

    <windowRules>
      <windowRule identifier="lxqt-panel" matchOnce="true">
        <skipTaskbar>yes</skipTaskbar>
        <action name="MoveTo" x="0" y="0" />
        <action name="ToggleAlwaysOnTop"/>
      </windowRule>
    </windowRules>