xcb and xlib platform plugins: Implement support for more buttons on "mouse" devices (up to 31 buttons).

Qt::MouseButton (and the corresponding flags) are expanded into a 32-bit field, supporting name/value bits for up to 31 buttons. Following this prerequisite namespace change, the plugins are enhanced to translate high-numbered buttons into the new Qt::MouseButton bit fields. And finally, a one-line change is made in qguiapplication.cpp, allowing button numbers above "XButton2" (AKA Button #9, AKA the "Forward" Button) to pass through for higher-level processing.

This code is all done, and tested with a widget-based test program. Other plugins (e.g., wayland) must be enhanced in subsequent tasks.