osd: add window-switcher field content types (#1623)

osd: add window-switcher field content types (#1623)

...`workspace`, `state`, `type_short` and `output`.

Example usage:

    <windowSwitcher allWorkspaces="yes">
      <fields>
        <field content="workspace" width="5%" />
        <field content="state" width="3%" />
        <field content="type_short" width="3%" />
        <field content="output" width="9%" />
        <field content="identifier" width="30%" />
        <field content="title" width="50%" />
      </fields>
    </windowSwitcher>