qt: remove monitoreddatamapper

We haven't used the viewModified signal in ages, so we can use a normal
QDataWidgetMapper.
fix build with libc++ after 85c579e
Merge pull request #5128

3a757c5 fix build with libc++ after 85c579e (Cory Fields)

Signed-off-by: Gavin Andresen <gavinandresen@gmail.com>