Bug 1051100 - [B2G] [Homescreen] Multiple lines are displayed when moving apps to the bottom of the Homescreen

* Before adding a horizontal divider in edit mode on the vertical homescreen, check if one is needed.
Merge pull request #23993 from begeeben/1051100_multiple_lines_are_displayed

Bug 1051100 - [B2G] [Homescreen] Multiple lines are displayed when moving apps to the bottom of the Homescreen, r=Chris