QWindow: add new function: QWindow::clearFocus()

The motivation for the function is that we need a way (for
iOS/Android) to tell the currect window to remove focus from the
focus object when the user hides the input panel. Otherwise, if the
focus object is e.g a line edit, the cursor will continue to blink
inside it, which is wrong.

Activating another window (or the top-level window) is not feasible
as this can cause the activated window to immediatly give focus to
a child (which is the case for QQuickWindow). And Telling the window
to deactivate can currently cause the whole application to deactivate.

Change-Id: I7c6447fd793896f242a99a2663c1e5cb8c948f7c
QGuiApplication: clear focus when input panel hides

When the input panel hides, we should clear focus from the
current focus object. Otherwise, if the focus object is e.g
a line edit, the cursor will continue to blink inside it, which
is wrong (on both iOS/Android).

Change-Id: I45ba311bc4dfb4cad8518ec2fc9de9e53192c847