vcl: remove unnecessary headers in clipping.cxx

Change-Id: I9b654b9ce51509bb84c7ac22c0e008e47adf1921
vcl: remove unnecessary headers from window.cxx

Change-Id: I80d9d456dc9cf99bf9bfa05571d42774f43ce577
vcl: tabify WindowImpl

Change-Id: Id348b2d2d4003777172df1c921e7ea6db5c11762
vcl: rearrange function location in window.cxx

Change-Id: I37db5776a3cb28de2c4fd7938566f7f3b2e2b8b2