Fixes numerous issues:
- the default behaviour had changed. It should disable force font DPI,
not set it to 96
- disabling didn't work, we'd set the value to 0, which would update
the spinbox, which has a minimum which sets the value to 96 (we were
warned of this with a binding loop error)
- toggling the checkbox didn't enable the apply button
New code fixes that and is more declarative too.