BUG: Specify application-modality when opening dialogs as application-modal.

Modal Traits UIs are always opened as application-modal: the call to `exec_()` always blocks the entire application. However, we have been incorrectly specifying the window-modal flag instead. To make an actually-window-modal dialog, one needs to work asynchronously by listening to the `finished()` slot of the `QDialog` and doing all of the follow-up code there; then one calls `open()` instead of `exec_()` to avoid blocking the whole application. We don't do this.

The practical change is one of styling on the Mac platform. Window-modal dialogs appear as drop-down sheets from their parent window, and not independent windows with their own titlebar. These are intended for small dialogs, like "OK/Cancel" dialogs, while most Traits UI Views do not fit this styling very well . Notably, with this change, a [Tasks application preferences dialog](http://docs.enthought.com/envisage/tasks_user_manual/extensibility.html#creating-a-preferences-dialog) will no longer appear as a sheet.