Montage previews cannot be closed on macOS

[cbrnr]

When previewing a montage (Edit – Set montage..., select any montage and click on View), the montage previews that open in a separate window cannot be closed as long as the Set montage dialog is still open.

[hofaflo]

Cannot reproduce on Win10, multiple montage preview windows can be closed independent of the "Set montag" dialog's state 🤔

[cbrnr]

A nice solution would be to include the previews in the montage dialog, perhaps with a collapsible "Details" button.

This example shows how to embed a Matplotlib figure in a QWidget:

https://matplotlib.org/stable/gallery/user_interfaces/embedding_in_qt_sgskip.html