gtkplacessidebar: Show busy spinner in sidebar row during mount ops
There is currently no feedback during mounting operations triggered via the places sidebar. This merge request adds a spinner to the sidebar row, similarly to the spinner already shown in the places view during mount operations.
There is special need for this with the new support for unlocking TrueCrypt/VeraCrypt volumes, which takes a long time.
Corresponding gtk3 merge request: !260 (merged)