Previously, the constructor of GameConfigEdit wasn't doing anything with
the passed in parent pointer. This is dangerous because it can result in
memory being leaked in certain scenarios. It can also affect layout
decisions made by the parent. Instead, pass it through to the base class.
Current usages of the class pass in nullptr as the parent, so this is a
safe change to make with regards to the class hierarchy.
While we're at it, we can std::move the passed in QString into the class
member, allowing calling code to move strings into the constructor,
avoiding copies.
