Make "Group by" groups foldable

[qithron]

Checklist

• [X] I made sure that there are no existing issues - open or closed - to which I could contribute my information.
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• [X] I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• [X] I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• [X] 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