map: marker display follows page

Instead of following the current selection, the map now follows the page.
Thus, all marker pins now remain on the map when the selection changes.

Introduce a new highlighted state (different from the selected state.)
The highlighted visible state takes priority over the selected state.

A marker group is shown as selected when it contains a selected marker.

When clicking on a marker group, all grouped photos (and only those) will
be selected in the library window.