Allow switching raster layers on and off

[kebekus]

Users want to switch raster layers on and off in flight. This request is related to the following discussion on the french gliding forum, where two pilots produces a wonderful map for the alps that shows minimum heights required to go to the next airfield

https://www.volavoile.net/topic/14449-fond-de-carte-custom-pour-les-alpes/

Ideally, users should be able to install several raster maps, and to choose between them.