This adds a new custom property widget which displays a button
on the properties panel and will invoke the property's command
when clicked.

For example:

  <Property name="New Instance Button"
            command="New"
            panel_widget="command_button">
  </Property>

Will show a button that when clicked will call the "New()" method
on the proxy.

Change-Id: Iba4a9af38670cff1e641dc0ba970b69a99adb229