And isEnabled() and isReadOnly() methods to Navigator

We have a lot of tests which result in javascript making things enabled and disabled. Rather than:

then:
!button.disabled

when:
xyz

then:
button.disabled

when:
abc

then:
!button.disabled
button1.disabled

I have added the method .enabled onto the navigator to allow more readable tests.

I have also done the same thing for readOnly (editable) to maintain consistency.