Allow rmgen terrain with actors

[elexis]

On survival of the fittest, we would like to place actor trees rather than actual trees, so players can't even attempt to gather from these trees.

However due to the TERRAIN_SEPARATOR being "|", we can't use special filter templates such as "actor|treetemplate" for terrain.

This ought to be solved by not changing template name strings, but passing arrays to the Terrain.

[elexis]

Another use case for the addition of actor entities to terrain entities is the addition of mushrooms in r21099 to birch forests.

[FeXoR]

Worked on in ​D1589