Use .isDisabled() instead of .disabled to support inheriting

If a widget ends up mixing in OO.ui.ItemWidget then .isDisabled() will no
longer be equivalent to .disabled, making inherited disabled states not
work.

Changes:
* Convert all uses of .disabled to .isDisabled()

Bug: 64759
Change-Id: Ia3bcd65a41e39757bdc926473c8120a161f6f626
Merge "Use .isDisabled() instead of .disabled to support inheriting"