Major improvement for MTB-Theme

Michael from Heidelberg contributed a MTB-Theme based on the andromaps_mtb where he invested lots of afford to bring in his real-world experience with MTB-cycling/Openandromaps.

The most significant change is that he avoids yellow rendering of paths with no MTB:Scale tagged. In addition he invested lots of afford to adjust the rendering to the different zoom-levels.
For me this Theme is a major improvement so I decided to serve it as standard for MTB (andromaps_mtb.xml).

The new legend and theme is already online: [klick]

Best regards, Christian