Optimize Overlay behaviour for better speed and less battery use and better usability

This request covers two aspects:

1) Do not calculate/try to display base map items when overlay has 100% visibility

This change has obvious benefits, reducing battery consumption and increase responsiveness.

And it does not imply re-architecting the map management ...

For those who did not follow the "raster versus vector map for low zoom levels" dispute: this will optimize the display speed at low zoom levels for the workaround that joelec found.

FYI - here are the map sizes for the workaround:

- zoom 8: 450 MB - this is equivalent to current vector map level, uses more than its 10 MB, obviously, but gives you a useful map instead, and first of all: one of your choice

- zoom 9: 800 MB

- zoom 10: 2.2 GB

Now #2) Currently, above maps need to include a layer of transparent tiles, one level above max zoomlevel.

A remedy is to add a choice to the Overlay definition screen "Zoom beyond max".

If checked, it will behave as today, bluring the overlay more and more while growing zoom levels

If un-checked, Locus will not display the Overlay map at all, once beyond the max zoom in the map file.

This will reduce the above file sizes by about 10%, and it will give more control to users to fit the needs of their Overlay use cases.