Add engine support to load mods from config and restart into mods.

Restructure mod mounting code a bit to make it reusable, and use it for
replays. Fixes #2703.

This was SVN commit r15676.

source/scriptinterface/ScriptConversions.cpp

@@ -421,6 +421,7 @@ VECTOR(u32)
 VECTOR(u16)
 VECTOR(std::string)
 VECTOR(std::wstring)
+VECTOR(CStr8)
 VECTOR(CScriptValRooted)