Make spacing element on panel bindable

[andrew-vos]

It would be beneficial to have the spacing element on on panel bindable:

  panel_abilities_available_list:
    type: fragment
    content:
      type: panel
      padding: 20
      elements:
        - type: panel
          layout: horizontal
          spacing: ~ui_ingame_abilities_panel_spacing()
          background: ~ui_ingame_abilities_panel_is_hovered() then $theme.resources.control_bg_light else $theme.resources.control_bg
          yoffset: ~ui_ingame_abilities_panel_yoffset()
          elements: @@ ~obj_player_state.available_skills
          template:
            type: ability_definition
          events:
            on_hover_changed:
              - @@ e => ~ui_ingame_abilities_panel_hover_changed(e.hover)

This would allow the following sample logic to be implemented:

• when I hover on the card deck in the corner I want spacing to change a bit and cards raise up and highlight -- I do already have this logic except for the spacing,
• when I click on the hovered deck the card would slide to the right making a "hand" -- I thought this potentially be achieved by animating the spacing value e.g. from -130 to 10

Ideally spacing should be possible to animate to implement nice sliding effect