Add: precondition checks to functions that work with both valid company and deity

These are functions that either use ScriptObject::Command or ScriptObject::GetCompany.
This is a bit over-protective, but having the check everywhere makes it easier to
validate that no check is missing automatically instead of by review.

At this moment these checks will not do anything useful, as either IsValid or
IsDeity from ScriptCompanyMode returns true, but that will change later.

Codechange: use GSCompanyMode::IsValid, IsDeity, and the precondition helpers

Direct 1:1 replacements in the code, and comments now refer to either
GSCompanyMode::IsValid or GSCompanyMode::IsDeity instead of several variations
on "company mode active" or "no company mode active".

Add: Allow GameScript to demolish without a company

This allows GameScripts to shrink towns as well as grow them.

(svn r25213) -Fix [FS#5537]: clarify on which tiles IsDesertTile and IsSnowTile work, i.e. the ones without infrastructure or buildings
-Feature: introduce GetTerrainType which allows one to get that information for tiles with buildings and infrastructure as well