Add 'valid' event to MediaSizeWidget

Add a valid event whenever the validity status of the media size
widget changes. Also, slightly change the placement of the 'change'
event to make more sense; it should be emitted when actual changes
were made, which is in 'setCurrentDimensions'

Bug: 72155
Change-Id: I925a9fa50ac56b015fd3c4d47a0add9589e78f4f
Merge "Add 'valid' event to MediaSizeWidget"