osd: add window-switcher custom field (#1670)

osd: add window-switcher custom field (#1670)

Add custom field with subset of printf style formatting
to replace the original field formats.

Example:

    <windowSwitcher preview="no" outlines="no" allWorkspaces="yes">
      <fields>
        <field content="custom" format="foobar %b %3s %-10o %-20W %-10i%t" width="100%" />
      </fields>
    </windowSwitcher>

Mono space font recommended. May need OSD width adjusted

Co-authored-by: @Consolatis (based on work done by them)