Merge #tracks-tunnels and #footbikecycle-tunnels into #tunnels

The layers #tracks-tunnels and #footbikecycle-tunnels are merged into #tunnels.
This is a preliminary step to merging the definitions for normal roads, tunnels, and bridges.

Consequences for rendering:
Rendering order of footways etc., paths and tracks is now determined by z_order instead of explicitly defining footways etc., paths and tracks to be rendered lower.
Therefore, minor order changes between service roads, footways etc, paths, tracks in tunnels might occur.