[framework]: changing a slot attribute does not update the corresponding element property

[pskelin]

Bug Description

When children a processed, the existing slot attributes are taken correctly and child nodes are assigned to a component property

For example

<my-parent>
  <my-child slot="header">

then in the parent element

@slot
header;

this.header returns the assigned node correctly.

The issue is when the <my-child> is rendered without a slot attribute and the attribute is added later via child.setAttribute("slot", "header"), this.header is not updated as it should be.

cc: @aborjinik