introduce new configuration value FIND_IN_AREA_LIMIT

It limits amount of pictures simultaneously displayed on the map

Signed-off-by: Radek Dostal <rd@radekdostal.com>
Improve picture distribution for Issue #14

Changing size of grouping rectangle, so exactly FIND_IN_AREA_LIMIT fits
on the screen.

Signed-off-by: Radek Dostal <rd@radekdostal.com>