Make QUndoModel public, move to Qt GUI

QUndoModel should be made public in Qt GUI so that Qt Quick applications can use it.

Some work was already done towards making the undo framework available to non-widgets applications:

QTBUG-40040

We should probably remove the following (very specific) API from it and let QUndoView create it's own model that has it instead:

void setCleanIcon(const QIcon &icon);
QIcon cleanIcon() const;