Utils: let SubmitEditorWidget handle empty description

SubmitEditorWidget::canSubmit() now return false if the description
field is empty. This avoids dummy code duplication in VCS plugins.

Change-Id: I11e4928e8eed432ccee6fa1f3516f0179a54625c
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-by: Bill King <bill.king@nokia.com>