Remove a few now unused files

CMakeLists.txt

src/ChatPage.cpp

src/ChatPage.h

src/MainWindow.cpp

src/MainWindow.h

src/UserSettingsPage.cpp

src/WelcomePage.cpp

src/WelcomePage.h

src/ui/DropShadow.cpp

src/ui/DropShadow.h

src/ui/FlatButton.cpp

src/ui/FlatButton.h

src/ui/Label.cpp

src/ui/Label.h

src/ui/LoadingIndicator.cpp

src/ui/LoadingIndicator.h

src/ui/Menu.h

src/ui/OverlayModal.cpp

src/ui/OverlayModal.h

src/ui/Painter.h