Use central place to show error dialogs by means of some ErrorManager

That would avoid having an additional state and two parameters per dialog we want to show.

I think @seb already had a working implementation for #81 (closed).

added Design T: feature request labels

mentioned in merge request !107 (merged)

yes, here's the draft: https://code.briarproject.org/briar/briar-desktop/-/tree/81-error-manager

This will also help to implement scrim over the whole app, including the ExpirationDialog.

mentioned in merge request !263 (merged)