KML Overlay : Add multiple files simultaneously

[Aryan]

When Add KML Files is clicked, the directory box which opens allows only single KML File to be added at a time. It is bothersome, and multiple KML Files in the same directory should be allowed to be added altogether.

[Joern Ungermann]

fixes #559 and #560 - Adding and Loading Multiple KML Files simultaneously

→ <<cset 1cc4d305e10a>>