Implement proper font handling

fonts/mono_dejavu_sans_28.xml

fonts/mono_dejavu_sans_280.png

fonts/mono_dejavu_sans_4.xml

fonts/mono_dejavu_sans_40.png

fonts/mono_dejavu_sans_6.xml

fonts/mono_dejavu_sans_60.png

fonts/mono_dejavu_sans_8.xml

fonts/mono_dejavu_sans_80.png

fonts/mono_dejavu_sans_9.xml

fonts/mono_dejavu_sans_90.png

minetest.conf.example

src/CMakeLists.txt

src/client.cpp

src/constants.h

src/defaultsettings.cpp

src/defaultsettings.h

src/drawscene.cpp

src/drawscene.h

src/fontengine.cpp

src/fontengine.h