Selecting and unselecting map markers is now done entirely through the
ShumateMarkerLayer rather than the individual ShumateMarkers. This makes
the code simpler and matches how e.g. GtkListBox works.

Some notable changes:
- The ShumateMarker methods for selecting and unselecting a marker have been
  removed and replaced with methods in ShumateMarkerLayer.
- Setting the selection mode to GTK_SELECTION_NONE no longer sets the
  selectable property of children to FALSE.
- Added marker-selected and marker-unselected signals to
  ShumateMarkerLayer.
- Selecting a marker properly deselects the other markers in
  GTK_SELECTION_SINGLE mode.