Listen to widget changes in MediaDialog

Attach event listener to the alternate text and the media size widget
in the MediaDialog, so that when either of those are changed the
'Apply Changes' button is enabled.

Related to I88c3f3dcc7667b3ffdd2a8a8433cb47b82ba1531

Bug: 67684
Change-Id: I03efbed2f9b2909b055d96b9ae1e27b6d5e2bdbf