ProxyTextView: Emit validation-changed and do not work with ValueUnset

* Emit 'validation-changed' when it gets updated by hand, just like other
  widgets do.
* Remove the use of ValueUnset on ProxyTextView. If it's empty, an empty string
  will be returned on read. Although, one can still pass ValueUnset to update so
  it gets updated to "" (empty string).
* Simplify the code a little on update to avoid duplicates.

Change-Id: Ia97f92735582785115191b8c3d4698a58ef20275
Merge "ProxyTextView: Emit validation-changed and do not work with ValueUnset"