Verify if we can eradicate QKeyMapper class (it is a class in *_p.h file)

It looks that when moving from Qt4 to Qt5 the complexity of this class has been reduced to the minimum. In process of this some functionality have been lost, e.g QTBUG-27681. and most logic now is handled via QLocale. So QKeyMapper doesn't really do anything useful, just adds an extra layer of indirection (what could be done directly from qshortcutmap.cpp and qplatforminputcontext_p.h). See the attached sample WIP patch and https://codereview.qt-project.org/#/c/216060/