- 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.
- Approximate the drawn curve by a set of Bezier curves in using
js code from https://github.com/soswow/fit-curves.
The code has been slightly modified in order to make the linter
happy.