Addresses #7836: Noisy $fill widgets

[flibbles]

$fill widgets will get ignored when encountered by ts-raw slots

There. If anyone complains about the loss of the old behavior, I will eat my T-shirt and post it to YouTube.

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[rmunn]

Thank you; this is exactly how I wanted <$fill> widgets to work.

Here's a quick test case I did:

\widget $my.widget(label:"")
<$slot $name="header"/>
<fieldset>
   <legend>&nbsp;<$text text=<<label>>/>&nbsp;</legend>
   <$slot $name="ts-raw"/>
</fieldset>
\end $my.widget

<$my.widget label="important">
<$fill $name=header>My widget header</$fill>

content of widget
</$my.widget>

What that looks like in TW prerelease right now:

What it looks like with this PR applied: