[Mac] fix link label breakage

Commit e7586f3e35c1250116015a14547eccd18e3d1fd4 broke link labels on Mac since LinkLabelBackend is a subclass of LabelBackend, it needs to wrap the native widget in a CustomAlignedContainer too. This will make any subclass of LabelBackend wrap the native widget as long as it calls one of the base constructors.