DEMO: Use different UIs. Move existed ui to own directory.

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rename : demo/gui/CMakeLists.txt => demo/client-gtkmm3/CMakeLists.txt
rename : demo/gui/client-main-window.cpp => demo/client-gtkmm3/client-main-window.cpp
rename : demo/gui/client-main-window.hpp => demo/client-gtkmm3/client-main-window.hpp
rename : demo/gui/client-opt-dialog.cpp => demo/client-gtkmm3/client-opt-dialog.cpp
rename : demo/gui/client-opt-dialog.hpp => demo/client-gtkmm3/client-opt-dialog.hpp
rename : demo/gui/player-signup-dialog.cpp => demo/client-gtkmm3/player-signup-dialog.cpp
rename : demo/gui/player-signup-dialog.hpp => demo/client-gtkmm3/player-signup-dialog.hpp
rename : demo/gui/ptbe-demo-client.cpp => demo/client-gtkmm3/ptbe-demo-client.cpp
rename : demo/gui/ptbe-demo-widget.cpp => demo/client-gtkmm3/ptbe-demo-widget.cpp
rename : demo/gui/ptbe-demo-widget.hpp => demo/client-gtkmm3/ptbe-demo-widget.hpp