frontend: clear invalid color on DialogTab

markAsInvalid() sets the border to "red" in AbstractValidatedWidget.
If after marking something as invalid the same field is marked as valid,
the DialogTab.markAsValid() did not clear this border. Consequently, if some
validation failed, the side tabs stayed red forever.

Change-Id: Ia5e0f92112d62295aef9b82424f3dd5f8097ebc0
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>