On wall expandibility

[fabio]

Two issue found while playing on Fortress RMS:

• it's not possible to expand walls from the existing tower walls, I can start the wall building mode but I get the red wall and the message "you can't build wall here". However there are no terrain obstruction, indeed I can build wall not connected to existing towers at the same place;
• it should be possible to extend walls also if terminating wall towers are not available. In this case I simply can't start the wall building mode. I suppose this should also the vase when a tower is destroyed and I want to replace it;
• also it may be useful to be able to build a single wall tower, for example to complete the wall on Fortress RMS (missing wall towers), to replace an existing destroyed tower or also to build a single wall tower without walls around (it was discussed on the forum that wall towers will have not default arrow so it can be used as a cheaper alternative to the standard tower).

[Deiz]

In 12258:

Added engine-side obstruction group sanitizing, saving, and loading. Refs #1566

[Deiz]

The former issue should be solved for all cases, including both RMS maps and all scenarios that include walls (mostly for Iberian players).

I'll look at the latter issue later. It might be easy enough to fix for alpha 11, but I'll keep the milestone where it is until I've taken a proper look at the relevant code.