Having to rename project settings is rare, but when it does occur it can cause user confusion. In order to make compatibility more seamless this PR introduces two new GLOBAL_DEF functions,
GLOBAL_DEF_ALIAS(new_name, old_name, default)
GLOBAL_DEF_ALIAS_RST(new_name, old_name, default)
These are the same as the existing GLOBAL_DEF functions except that if the new setting is not found, it attempts to load from the old setting name. If the old setting is found, it stores it into the new setting, and then calls the regular GLOBAL_DEF functions.