Instruct the render process to change the text selection if it was
requested via an input method event. Raise the selectionChanged() signal
when all the corresponding input method properties are set.

Moreover, add back the remaining input method widget auto tests. The
updated tests are moved to the QWebEngineView tests since the
corresponding APIs (inputMethodQuery() and input event handling) are now
available via the QWebEngineView's focus proxy (aka RWHV).

Task-number: QTBUG-55766
Change-Id: Ia0022d5f38b31dd59b084ff42e4abc2780ae90ec
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>