Remove/deactivate click handler for the "add bookmark button" for a dossier until the add book mark operation has completed.

[ISN-Zurich]

Expected behaviour: additional clicks on the "add bookmark button" before the add bookmark operation completed should be ignored

[othbert]

to implement;

1. prevent multiple clicks by disabling the clickable state of the button
2. reinstate when the function to add the item to dossier has ended
3. modify add item function to check whether already exists before insert occurs, possibly just use replace into?

[zielperson]

so combine #150 with this?

0. pretty effects for clicking the button