move *Client.* to layers/client

--HG--
rename : gfx/layers/CanvasClient.cpp => gfx/layers/client/CanvasClient.cpp
rename : gfx/layers/CanvasClient.h => gfx/layers/client/CanvasClient.h
rename : gfx/layers/CompositableClient.cpp => gfx/layers/client/CompositableClient.cpp
rename : gfx/layers/CompositableClient.h => gfx/layers/client/CompositableClient.h
rename : gfx/layers/ContentClient.cpp => gfx/layers/client/ContentClient.cpp
rename : gfx/layers/ContentClient.h => gfx/layers/client/ContentClient.h
rename : gfx/layers/ImageClient.cpp => gfx/layers/client/ImageClient.cpp
rename : gfx/layers/ImageClient.h => gfx/layers/client/ImageClient.h
rename : gfx/layers/TextureClient.cpp => gfx/layers/client/TextureClient.cpp
rename : gfx/layers/TextureClient.h => gfx/layers/client/TextureClient.h