add function to start 'idle' mode - for use in the mobile app (rather than it forcing the idle timer)
the existing idle resume function can be used on returning to iitc

also, fix the long-standing bug on mobile where the map disappears after repeated touch panning/zooming. seems leaflet gets confused about zoom levels and ends up with a non-ingeger. when this is found, force it back to an ingeger zoom level when this happens