GPXLab should provide several kind of maps

Hello,

Currently GPXLab seems to only support one map at a time and it's not designed to change easily map source.

Default tile url is currently http://tile.openstreetmap.org/%1/%2/%3.png

Maybe supporting several kind of maps could be a nice improvement
like GPXSee
http://www.gpxsee.org/
https://github.com/tumic0/GPXSee-maps

Kind regard