Fix Calendar::updateNotebook event visibility updates

Broken by commit 6a183555. Earlier this was using QMultiHash::values(key) for updates but the commit made it loop over all the events, thus when some notebook was updating visibility, everything got updated accordingly.

On behalf of Pekka Vuorela from Jolla, I'm proposing this commit to the KF5 branch. As mentioned by @pvuorela, an earlier commit changed a for loop on values of a QMultiHash for a single key, into a for loop on all values (independantly of the key). This resulted in events of all notebooks changing visibility when changing the visibility of one single notebook.

The commit reestablish the loop on the values sharing the same key.