Allow triggers to support child definitions

[Pablete1234]

Upgrades trigger's attributes to properties, meaning they can be used either as attributes or children:

<actions>
  <trigger scope="match">
    <filter>
      <variable var="something">0</variable>
    </filter>
    <action>
      <set var="something" value="1"/>
      <set var="something_else" value="something+5"/>
    </action>
  </trigger>
</actions>

Equivalent to:

<filters>
  <variable id="something-is-0" var="something">0</variable>
</filters>
<actions>
  <action id="set-something-to-1" scope="match">
    <set var="something" value="1"/>
    <set var="something_else" value="something+5"/>
  </action>
  <trigger filter="something-is-0" action="set-something-to-1" scope="match"/>
</action>

In some situations it's better to just inline them than to have different ids for each filter and action

The PR has been tested and works as intended.