Add a popup when image is exported in Shared Notes activity

[llaske]

Shared Notes activity allow generation of the current work in the Journal by clicking on the dedicated icon in the toolbar (to the right on the screen capture below).

It will be nice to notify the user when this export is done.
The idea is to add a popup like in the QRCode activity.

The popup should be displayed using humane library and the message should be localized. See here the implementation in QRCode activity.

[llaske]

Fixed in #883