Add ClearSelection method #6375

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Idea from #6277, and at least widget.Entry and widget.Label have selection aspects, and other widgets might qualify as well as per #1395 since it requires no knowledge of internal selection details.

Is it possible to construct a solution with the existing API?

Calling SetText with the same text again could work but is a bit weird.

Describe the solution you'd like to see.

ClearSelection method for widget.Label and widget.Entry.