[PATCH] Undeletable entities

[leper]

For some maps having undeletable (and indestructible (which can be done by having a trigger using cmpArmour.SetInvulnerability() already) entities can enable some nice gameplay features.

Examples would be trade posts on the Silk Road map niektb is working on, or mercenary camps.

Below patch takes care of this, selecting entities and pressing del still opens up the delete dialog, but no deletion takes place.

[mimo]

Any reason for not doing as for mirages when pressing the del key, i.e. returning a tooltip explaining why you can't delete it and a different icon in the getinfo function, and adding a return in the execute function ? Apart from that, looks fine.

[leper]

In 17408:

Add support for undeletable entities. Fixes #3590.

[leper]

In 17416:

Fix test failure. Refs #3590.