[GUI] ask user for confirmation when recovering files

Pushing this button may be harmful if a user has not read the
content in the bubble notifications.

TODO: improve the messages and output in the detailed view

Change-Id: I9e9f201fe7b8f478e7d22676217d64f024cc4f45
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1072
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>