Using other map sources

Hello,

it will be a nice improvement to be able to use other map sources than Google map.

Maybe supporting WMTS url service could be a great idea.

https://github.com/tumic0/GPXSee-maps provide several kind of map source definitions.

Kind regards

PS : see https://gis.stackexchange.com/questions/201425/how-to-use-wmts-url-service-in-my-android-map-application-as-basemap