Add tooltips for back and forward buttons
GNOME HIG requires unlabeled buttons to have tooltips. Even an obvious button like "Forward" needs this for ease of use and accessibility reasons, as it does in Nautilus, Gitg and others apps...
Although the Calendar app has tabs that change the content, this doesn't take away the possibility of having general tooltips for the forward and backward buttons. For this reason, this merge request requests that tooltips be added.