Create a simplified way to call a subset

[marcoriesco]

Issue description

Is there any way to call user-defined subsets in a more simplified way?

Below, I use this method and then what would be a simplified method

<video ifSubset="video_gamelist:Ativo" name="md_video">
    <origin>0 0</origin>
    <pos>0 0</pos>
    <size>1 1</size>
    <delay ifSubset="video_gamelist_delay:0.5">0.5</delay>
    <delay ifSubset="video_gamelist_delay:1">1</delay>
    <delay ifSubset="video_gamelist_delay:1.5">1.5</delay>
    <delay ifSubset="video_gamelist_delay:2">2</delay>
    <delay ifSubset="video_gamelist_delay:2.5">2.5</delay>
    <delay ifSubset="video_gamelist_delay:3">3</delay>
    <delay ifSubset="video_gamelist_delay:4">5</delay>
    <delay ifSubset="video_gamelist_delay:5">5</delay>
    <effect>none</effect>
    <zIndex>16</zIndex>

    <storyboard event="activate">
        <animation property="opacity" from="0" to="1" duration="800" mode="easeIn" />
    </storyboard>
</video>

Perfect world:
<delay>${video_gamelist_delay}</delay>