Add ClearSelection method to Entry and Label

[uttam12331]

Fixes #6375.

Adds a ClearSelection() method to widget.Entry and widget.Label so callers can programmatically cancel an active text selection without needing knowledge of the internal selection state (the workaround previously was re-calling SetText with the same text).

• Entry.ClearSelection() and Label.ClearSelection() clear an active selection and refresh; both are no-ops when nothing is selected (or, for Label, when it isn't Selectable).
• Added tests TestEntry_ClearSelection and TestLabel_ClearSelection covering the selection-then-clear path and the no-op case.

Verified locally with go test ./widget/ (both tests pass), gofmt, and go vet ./widget/.

[andydotxyz]

Thanks for this, but since needs to target the next release as we don't add API in point releases.
The feature window is missed on 2.8 so please update to 2.9

[uttam12331]

Thanks for the review! Updated the Since: tags on both Entry.ClearSelection and Label.ClearSelection to 2.9.