Add "GseSavePath" env var for overriding emu save path

[wunnr]

Much like the existing env var "GseAppPath" that overrides the emu's program path. This new env var may make it easier for programs like Nucleus Coop or my own PartyDeck that launch multiple instances of games at a time with goldberg. The check for the env var seemed most appropriate in Local_Storage::get_user_appdata_path(), but you can let me know if it would be better off as part of a different function.

[universal963]

I don't think it is an appropriate location, since by judging the function name it should be used only to retrieve %appdata% directory. What about this function when parsing local_save:

gbe_fork/dll/settings_parser.cpp

Lines 607 to 619 in db0bc4c

	// user::saves::local_save_path
	static bool parse_local_save(std::string &save_path)
	{
	auto ptr = ini.GetValue("user::saves", "local_save_path");
	if (!ptr || !ptr[0]) return false;
	save_path = common_helpers::to_absolute(common_helpers::string_strip(ptr), Local_Storage::get_program_path());
	if (save_path.size() && save_path.back() != *PATH_SEPARATOR) {
	save_path.push_back(*PATH_SEPARATOR);
	}
	PRINT_DEBUG("using local save path '%s'", save_path.c_str());
	return true;
	}

[wunnr]

Moved to parse_local_save as requested