The tasks PreferencesDialog should support disabling the 'OK' button

The [PreferencesDialog](https://github.com/enthought/envisage/blob/master/envisage/ui/tasks/preferences_dialog.py) will call apply on all the preference panes when 'OK" is pressed. However some times it is useful to be able to veto closing the dialog because the current settings in a pane are invalid or partial

I think that one way of having this behavior is if each `PreferencePane` has a `veto_apply` Boolean trait that the `PreferencesDialog` can monitor to make a decision on closing the window and enabling/disabling the 'OK' button.

Start with envisage/ui/tasks/preferences_dialog.py and inspect how PreferencesDialog calls apply on each PreferencePane and manages the OK button. Trace the existing pane traits and dialog lifecycle, then define the veto behavior so invalid or partial settings disable OK and prevent closing or applying; done means all panes can consistently veto the action.

Written by the indexing model from the issue text.