Show anchor icons for heading inside panels

[yamgent]

Tell us about your environment

• MarkBind Version: 1.13.0

What did you do? Please include the actual source code causing the issue.
Followup from #426 (comment).

<panel header="# Some heading">
    ... Some content here ...
</panel>

What did you expect to happen?
An anchor icon appears right beside the heading.

What actually happened? Please include the actual, raw output.
No anchor icon show up beside the heading.

[damithc]

Funny thing: when I tried v1.13.0, for a brief period these anchors did show up, but now they don't. :-p Even when they did show up, the were useless because clicking on them would collapse the panel instead of resetting the URL bar.