Replace pause and message menu by formspec ones

builtin/mainmenu.lua

builtin/modmgr.lua

builtin/modstore.lua

doc/lua_api.txt

src/CMakeLists.txt

src/game.cpp

src/guiFormSpecMenu.cpp

src/guiFormSpecMenu.h

src/guiMessageMenu.cpp

src/guiMessageMenu.h

src/guiPauseMenu.cpp

src/guiPauseMenu.h

src/main.cpp

src/mainmenumanager.h