- As in the annotation layer, use percent instead of pixels as unit;
- handle the rotation of the editor layer in allowing editing when rotation
angle is not zero;
- the different editors are rotated counterclockwise in order to be usable
when the main page is itself rotated;
- add support for saving/printing rotated editors.
- Right now, we must select the tool, then click to select a page and
click to start drawing and it's a bit painful;
- so just create a new ink editor when we're hovering a page without one.
