Horizontal scrolling behavior in sidebar widgets is inconsistent

[jonpalmisc]

The following sidebar widgets offer horizontal scroll bars if their content is wider than the sidebar:

• Symbols list
• Variable list
• Tags
• Cross references

and these do not:

• Types view
• Stack view
• Strings list

We should make this behavior consistent by either making all sidebar widgets horizontally scrollable or making none of them horizontally scrollable.

[fuzyll]

As per #3466, cross references no longer provides a horizontal scroll bar. If my understanding is correct, this is because it was changed to have the final field on the right fit-to-width, which had the unintended consequence of removing the scroll bar when that field is wider than the width of the widget. If that's the case, that may point to the underlying cause for the other 3 mentioned above also behaving the same way.