Extra 'collapsible' class on collapsible li-treeitems

[nullester]

Hi,

Just for extra CSS styling, it would be nice to have an extra class on the collapsible li-items.

Before:

<ul class="idx" role="tree">
    <li class="open" role="treeitem">
    ...

After:

<ul class="idx" role="tree">
    <li class="collapsible open" role="treeitem">
    ...

Reason. I'm hiding the default ::marker's and using custom ::before's in an extended theme with up and down arrows to make it clear these items are collapsible/expandible. But if the Item is not collapsible, I want to keep the regular bullet icon.

Thanks!