expose savefig.transparent / savefig.dpi in the UI #7352

It would be nice if, after interactively selecting a filename where the figure will be saved, a second dialog popped up to let the user pick figure transparency, DPI, JPEG quailty if applicable, etc. (basically whatever kwargs fig.savefig takes).

Labeling this as QUI/Qt because the presence of formlayout should make this relatively easy to implement in this backend, but possibly nice to have in other backends too. (By the way perhaps we should just have a general "GUI" label?)