Add extra-fine pen stroke size

[jasle]

The current smallest pen stroke size works well for most situations, but it is still slightly too thick for users who prefer very fine handwriting. Introducing an extra-fine stroke size (around 1f–2f) would significantly improve precision.

As an alternative solution, providing a way for users to define custom stroke sizes would give full flexibility for different writing preferences.

Suggested improvement:

• Add one finer preset stroke size between 1f and 2f
  or
• Allow users to define custom stroke sizes

This would greatly enhance the handwriting experience for users who like very thin strokes.

[Ethran]

I feel like additional option will clutter the UI, and in most cases is not needed.
The issue should be addressed when changing UI will be implemented, issue #151 . For now, it won't be worked on.