Add vertical/horizontal spacing parameter to GridView component

The `GridView` component should support vertical and horizontal spacing between items in the grid, like most other native grid views. The lack of this feature is frankly embarrassing...

Currently the only 'workaround' we have is to add a gutter style margin to the delegates in the grid, which forces us to have the layout shown in the picture below.

Here the red box is what we want our cell dimensions to look like. The green boxes are how we have to set our cell dimensions to be in order to get the the items spaced correctly. And the red squiggly lines show the unwanted 'gutter' space at the bottom and right of the grid that we are forced to have due to the GridView not implementing any actual spacing feature.

In addition to the unwanted space, this workaround also causes scrolling issues when we call `positionViewAtIndex()` as the whole cell size is taken to account by the various modes of this function, whereas it should only be affecting our actual cell.