Codechange: [Script] Use helper function over direct casting CompanyID

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.

src/script/api/script_companymode.hpp

@@ -11,6 +11,7 @@
 #define SCRIPT_COMPANYMODE_HPP
 
 #include "script_object.hpp"
+#include "script_company.hpp"
 
 /**
  * Class to switch the current company.
@@ -30,7 +31,7 @@
  */
 class ScriptCompanyMode : public ScriptObject {
 private:
-	CompanyID last_company; ///< The previous company we were in.
+	::CompanyID last_company; ///< The previous company we were in.
 
 public:
 	/**
@@ -40,7 +41,7 @@ public:
 	 * @note When the instance is destroyed, it restores the company that was
 	 *   current when the instance was created!
 	 */
-	ScriptCompanyMode(SQInteger company);
+	ScriptCompanyMode(ScriptCompany::CompanyID company);
 
 	/**
 	 * Destroying this instance reset the company to that what it was