Allow entity selections with multiple keywords (aka garrison flag fix)

[historic_bruno]

Currently the engine allows setting an entity selection with CUnit::SetEntitySelection, which controls animation-dependent variations like "walk", but it's restricted to only one entity selection at a time - overriding previous selections. Philip suggests instead maintaining a std::map (in CCmpVisualActor?) and then passing the multiple selection keywords to CUnit. The map would look something like this:

{"animation":"walk", "civ":"hele", "damage":"medium"}

ICmpVisual would need to expose the new selection interface to scripts, so that for example GarrisonHolder can use it to set the garrison flag, instead of the current broken hack that uses an animation (because we can only have one animation at a time, it will switch chaotically between idle/walk and garrisoned).