Add method to center widgets & clean up unused headers

src/InviteeItem.cpp

src/InviteeItem.h

src/Utils.cpp

src/Utils.h

src/dialogs/CreateRoom.cpp

src/dialogs/JoinRoom.cpp

src/dialogs/LeaveRoom.cpp

src/dialogs/Logout.cpp

src/dialogs/ReCaptcha.cpp

src/dialogs/RoomSettings.cpp

src/dialogs/UserProfile.cpp