[libcalamares] Rename cPointerSetter

[libcalamares] Rename cPointerSetter

This class doesn't really set a pointer -- it is a scoped assignment
through a pointer, which **can** set a value on destruction (when
it leaves scope). Rename it.

While here, extend the API so that it can do an assignment to the
underlying object **now**, while also doing a scoped assignment
later when it leaves scope. This makes some code a bit easier
to read ("in this scope, X is now <v> and then it becomes <v'>")