Add css property to widgets

[matt1432]

This PR is basically a copy of this commit but for AGS instead of EWW.

It allows setting CSS of subnodes such as this scale in EWW:

(scale :value song_pos
       :min 0
       :max song_length
       :orientation "h"
       :onchange "playerctl -p Spot position {}"
       :css "highlight { background-color: ${button_accent}; }
             slider { background-color: ${button_accent}; }
             trough { background-color: rgba(${button_accent}, 0.4); }")

It would be like this in AGS:

export const PositionSlider = (player, params) => Slider({
    ...params,
    className: 'position-slider',
    css: `highlight { background-color: ${button_accent}; }
          slider { background-color: ${button_accent}; }
          trough { background-color: ${button_accent}; }`,
    hexpand: true,
    drawValue: false,
...

[matt1432]

@Aylur Is there anything missing before you'd want to merge this PR?