canvas manager views

this is a small hack that allows for switching between a normal,
scrolling canvas manager and a null canvas manager

The idea is that the canvas manager handles the canvas and most window
events while the DocView controls which canvas manager is loaded
move page geometry attributes to roles
remove comment
refactor canvas coord function
canvas size function
page picker canvas manager

Only implemented the page placement so far.

Next step is to work on the rendering. This will require a sending a
zoom-level to the render thread based on the page column width.
remove trailing space