Multi-column text layout

[matus-z-ceros]

Add support for multi-column text layout. This would be a per-paragraph value.

If we design a column layout with equal length columns, we can describe the layout with three/four parameters:

• columns - the number of columns
• gutter - the space between the columns, in pixels
• padding(s) - the space between the columns content and text frame, in pixels

"text": {
    ...
    "layout": {
        "columns": 2,
        "gutter": 3,
        "padding": null,
        "paddings": [1,2,1,2]
    }
},