In Imagery Preferences add a button to help find and delete obsolete entries

[jidanni]

In Imagery Preferences please add a button that would comb for obsolete
entries like the first of

    <map>
      <tag key='name' value='MapBox Satellite'/>
      <tag key='type' value='tms'/>
      <tag key='url' value='http://{switch:a,b,c}.tiles.mapbox.com/v3/openstreetmap.map-4wvf9l0l/{zoom}/{x}/{y}.png'/>
      <tag key='max-zoom' value='17'/>
      <tag key='projections' value=''/>
      <tag key='icon' value='data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABgAAAAYCAYAAADgdz34AAAYJWlDQ1BJQ0MgUHJvZmls
    </map>
    <map>
      <tag key='name' value='Mapbox Satellite'/>
      <tag key='id' value='Mapbox'/>
      <tag key='type' value='tms'/>
      <tag key='url' value='https://{switch:a,b,c,d}.tiles.mapbox.com/styles/v1/openstreetmap/cj8gojt0i1eau2rnn7q4mdgu7
      <tag key='attribution-text' value='Terms &amp; Feedback'/>
      <tag key='attribution-url' value='http://www.mapbox.com/about/maps/'/>
      <tag key='permission-reference-url' value='http://wiki.openstreetmap.org/wiki/Vertical_Aerial_Photographs#Digital
      <tag key='max-zoom' value='22'/>
      <tag key='cookies' value=''/>
      <tag key='projections' value=''/>
      <tag key='icon' value='data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAMAAABEpIrGAAAAhFBMVEVHcEz///9CZPtp...

as seen in ~/.josm/preferences.xml .

Here we see a lack of 'id' etc. tags, as well as a duplicate 'name' etc.
hints...

So perhaps this comb function could "mark" them, and then the user could
hit the trash can button to delete them all.

One finds the obsolete entries came from the obsolete cache files in #15951.

But removing the obsolete cache files and restarting JOSM doesn't help
to remove the obsolete entries.

OK maybe that is what the white entries in #15949 are.

In this case there should be a "mark all white entries" button, so one
could subsequently delete them all.

And then there are also white ones in the upper box, above the "V Activate" button...