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

[rkern]

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 will no longer appear as a sheet.

[corranwebster]

This is something that @jasonmccampbell should be aware of, as it will impact the Canopy application, hopefully for the better.

[rkern]

It does fix the problem we were having with floating dock panes. The free-standing modal dialog is raised above them, unlike the sheet.

[jwiggins]

👍

[pankajp]

👍