api changes to layout grid

[JannesP]

basically what I wanted is something like:

grid: {
    row_weights: [ float, float, float, ... ], //weights for all rows or null/not defined (same for all)
    column_weights: [ float, float, ... ], //weights for all columns or null/not defined (same for all)
    elements: [ ... ]
}

and remove the weights from the elements itself because this makes much more sense for the host and is quite implementation specific on android