[Regression: 4 - 5&6] QCompleter can not use input method when completer popup is visible on Linux

Can be easily tested with the offical completer example:
https://code.qt.io/cgit/qt/qtbase.git/tree/examples/widgets/tools/completer/resources?h=6.4

Basically the issue is that, unlike on Windows, on Linux/X11, focusObject is changed to the popup's QListView when the popup is visible. QListView is not the correct focusObject and it's an input method disabled object. In this case and input method will simply ignore any key event.