TextInput: Update text deselection on focus out event

The text selection should not be cleared when the focus
out event received has one the following reasons:
- Qt::ActiveWindowFocusReason
- Qt::PopupFocusReason

When the user opens a menu or navigates to another window,
the eventual selection should not be clearer.

This also makes the behavior consistent with TextEdit.

Change-Id: Ibc6242cb2f8207cf5281925c8e20b88394f21eea
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>