Octree is a tree, that's used for color quantization, so it's
better to move it there. Headers are also moved to bitmap
subfolder so it's better organized.

Change-Id: I2b84a5469c1479cf0a060ba8eb46591dabab2883
Reviewed-on: https://gerrit.libreoffice.org/70726
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>