Fix emitting selectionChanged signal for non-user text selection change

This fixes the case when the text selection is triggered by JavaScript.
Text selection changes triggered by IME composition text replecement are
ignored. Test has been added for mouse selection clipboard: non-user
text selection should not update the clipboard.

Pulls in Chromium changes:
  3deea95 Update TextSelection for non-user initiated events

Task-number: QTBUG-53134
Task-number: QTBUG-60381
Change-Id: Ib94f57a2aa61248fba75f595245fb388b9609b6c
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>