Handle click outside of import process notification

[Korb]

Currently, after taping outside the message that an import is in progress, the message itself disappears and the application stops responding; The OS suggests closing it as unresponsive.

I suggest that in case of a tap outside the import progress notification, ask the user whether he wants to interrupt the import or continue it? Similar behavior seems logical to me with the Back gesture.

Another option for the application's behavior: in principle, it will not respond to taps on the screen outside the Cancel button on the import progress notification.

[TheLastProject]

As written in #1978 as well, the real issue that needs fixing is #513.

Given the current implementation has to go, I'm not really interested in adding more features, so blocking cancelling the import/export seems like the most logical quick hacky workaround for now.