Remove QT_KEYPAD_NAVIGATION

QT_KEYPAD_NAVIGATION is a define that is supposed to be set manually, when configuring & building Qt - it is not part of the Qt 5 or Qt 6 feature system.

It makes QApplication::setNavigationMode() available, which allows using QWidget elements with keypad (mouse_up, mouse_down ...) only.

Anyhow, the last big user of this was apparently Symbian. Nowadays it's documented in some places to be useful for "Embedded Linux", in other places as not supported at all. Anyhow, neither Qt 6.3 nor Qt 5.15 does build cleanly with configure -DQT_KEYPAD_NAVIGATION.

We should therefore remove QT_KEYPAD_NAVIGATION from the sources.