A Source is either a CompanyID (Headquarters), IndustryID or TownID.
When making those types stronger a lot of casts would be needed, but
with these simple helpers the intent is shown more clearly.
In the script's API `COMPANY_INVALID` has a value of -1, whereas the internal
game's `INVALID_COMPANY` has a value of 255. Since the script's API also has
a `COMPANY_SPECTATOR` with a value of 255, these enumerations cannot be easily
reconciled by casting. As such, replace all casts in the script API with
either ScriptCompany::FromScriptCompanyID or ScriptCompany::ToScriptCompanyID.
Also make clear whether CompanyID is ::CompanyID or ScriptCompany::CompanyID
by using either one of those over CompanyID in the script's API.
Command functions are those that call ScriptObject::Command, and functions
with company access are any that call 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.
