Refactor the eraser tool into 3 possible implementations

[maoschanz]

Use case

While the "fill"/"paint bucket" feature still sucks, its "remove color" option works very well, and is soooooo useful to cut out a background

It would be more intuitive and accessible to list that option in the eraser instead

Suggested solution

• On the same model as brushes, split the eraser into several files
  • the current one
  • an abstract thing
  • its "pencil" implementation
  • its "rectangle" implementation
• persist the value of the last used eraser mode
• Add the "remove color" implementation
  • in the UI ("magic" icon)
  • re-implement the existing behavior, using the color at the "release" signal
  • improvement: use a list of RGBs along the path of the pointer instead
• remove the feature from the "paint" tool

Possible drawbacks

The behavior of the #46 regarding the tool has to change, or to disappear

Possible alternatives

Distinct tools, but they might be too specialized?