Add option to filter images drawn on map

This reduces the images drawn on the map to the current collection/
filter, increasing the performance and reducing the mess on the map when
many images are geotagged within the current map section.
Also, changing map preferences doesn't require a restart any more.
use memory.collected_images for map view filter
removed superfluous loop index
introduce signal DT_SIGNAL_PREFERENCES_CHANGE
Small cleanups of the latest maps changes

Take care of
- typos,
- formatting,
- the unused proxy function,
- make the generated SQL query less ugly