QTabletEvent modifiers aren't set on linux

Since the recent refactoring of Qt regarding the synthesis of mouse events moved in the abstract code of the input management, on Linux there is 2 issues:

• The first one is that we don't receive MouseMoveEvent when hovering above a tablet. This will be fixed by this task: https://bugreports.qt.io/browse/QTBUG-26116. In the meantime the attached patch fix the issue.
• Modifiers aren't set by the xcb integration when calling the QWindowSystemInterface::handleTabletEvent() method.