Allow multiple KML imports

[rmackay9]

MP supports importing a KML file (which is very useful). It would be nice if multiple KML files could be included and they appeared on the MP map in different colours. Below shows MP showing all in purple while Google Earth shows them in different colours.

It might be useful if this list of KML files was persisted, loaded, saved, etc. One suggestion was to add a new item to the Mission/Fence/Rally drop-down called "Overlay" and then allow the maintenance of these items in the command list at the bottom.

This is an important feature for BVLOS pilots it seems.

FYI @tridge