Allow opening a dialog or popup part-modal

[danielwiehl]

Add support for opening a dialog or a popup modal to its part, similar to the view modality when opening a dialog or a popup in a view. Closing the part should hide the dialog or popup. Opening it from a peripheral part should position it in the center of the application.

[danielwiehl]

To consider:
Maybe, it is not necessary to support part modality at all because the available space is limited. This may also apply to views located in the peripheral area. It would make more sense to open dialogs from within peripheral views application modal.