Split off DesktopAffine class, add tests

Split off the class DesktopAffine and the enum CanvasFlip into separate header files. This reduces the length of desktop.h from 579 to 485 lines — a reduction of more than 16%.

These entities are moved to the Inkscape namespace. The enum CanvasFlip is converted to enum class. A boolean member variable of SPDesktop is reordered in order to reduce object padding.

The includes in the header desktop.h are sorted and cleaned up.

Unit tests are added for the class Inkscape::DesktopAffine.