The overflow layout sit at the bottom of a cell to display "Other X events"
when there is no room to accomodate more.
The issue is that the overflow layout does not have margin property like
other views(layouts) in the cell, so instead it appears either too far from or
too close to the border depending on the text it accomodates.
We can fix this by setting the margin property.
https://bugzilla.gnome.com/show_bug.cgi?id=7771581
