report-gtk: confirm the ask dialogs on 'Enter'

This commit adds a convinient way of closing an ask dialog. If user
enters some text, hitting 'Enter' key closes the dialog. Closing the
dialog with empty input text is not allowed because it is simmilar to
clicking 'Cancel' button.

Closes rhbz#1075589

Signed-off-by: Jakub Filak <jfilak@redhat.com>