also clean-up and 'standardize' the include sections of canvas' source code

Change-Id: Ib437a41ea41ae2ea6528f755ae5b2828775aaf73
Reviewed-on: https://gerrit.libreoffice.org/17633Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>