Switch for one/multiple selection in background layers

[JakubPelka]

Today: While selecting background layer it is now impossible to select more backgrounds than 1. It is possible to change the transparency of the layer with a slider - but it is useless now as it is impossible to blend 2 (or more) backgrounds.

Idea/Need: It would be good to have a possibility to select more backgrounds and blend them with transparency slider (even one that now exists).

Solution: Create a switch that will be changing between single selection (like it is now) and multi selection - like it is in standard layers panel. So the user can define if it wants to work same way as is working now, but will have possibility to start topo and orto in the same time and mix those 2 backgrounds if needed. Mix will be performed with existing transparency slider.

Temporary solution: Some layers might be moved from backgrounds to standard layers. And than it is possible to achieve resultat