Allow adding of TMS in the AddLayers dialog

This allows adding TMS services that support the map's projection through the AddLayers dialog. Tested with:

https://geoinformatics.aalto.fi/mapproxy/tms
http://hiu-maps.net/hot/1.0.0/basemap_a_v1/

If a TMS does support listing its tilesets, such as the last url, the user can provide a url to a single tileset to work around it.