Implement freezing the projector window

doc/katarakt.txt

share/katarakt.ini

src/beamerwindow.cpp

src/beamerwindow.h

src/canvas.cpp

src/config.cpp

src/layout/gridlayout.cpp

src/layout/presenterlayout.cpp

src/layout/singlelayout.cpp

src/resourcemanager.cpp

src/resourcemanager.h

src/viewer.cpp

src/viewer.h

src/worker.cpp