Bug 1051100 - [B2G] [Homescreen] Multiple lines are displayed when movin...

...g apps to the bottom of the Homescreen

* Before adding a horizontal divider on the vertical homescreen, check if one is needed.
Can you instead call ```grid.removeUntilDivider()```? It should clean things up a bit and remove some duplicate code.

Seems like we don't have an easy way to get to it, so if you don't want to move it into gridView, you should be able to access it by ```this.gridView.element.removeUntilDivider()```.