Add option to display workspace sidebar horizontally

[matthewdias]

When using a grid with few rows and many columns, the workspace sidebar in the activities overview gets pretty crowded. It would be useful to be able to set it to display above or below the windows overview.

[jcmonteiro]

@matthewdias take a look at my pull request #67. See if it suits you.

[matthewdias]

lgtm layout wise

[sanwablo]

Could there also be the option to show the workspace overview on the left and on the top?

I personally would prefer it on the left, as the pointer is on that side of the screen after launching activities with the mouse.