Make "Group by" groups foldable

[qithron]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
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• This issue contains only one feature request.
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Feature description

Short-clicking on group header fold the group. Related to #170.

Why do you want this feature?

Kill necessary scrolling to the next group in case of current displayed group has tremendous amount of items. Especially when "Group by" set to "File type".

Additional information

No response