Add a relative brush size option

[CactiChameleon9]

Relative brush sizing can be annoying/difficult in certain situations, such as when trying to draw a picture with consistent line width.

This PR adds an option to disable relative brush size (enabled by default to preserve normal behaviour)

Superseded by #9 (due to de-forking of the fossify apps)

[naveensingh]

These PRs are being closed automatically by GitHub or some glitch. I'm not closing them.

[naveensingh]

@CactiChameleon9

I detached all the forked Fossify repositories and as a result, PRs originating from SMT forks are being closed automatically. Please consider the following solution:

• Fork the new repository from Fossify.
• Add your SMT fork as a remote and merge your changes.
• Raise a new PR using the newly forked Fossify repository.

Sorry for the inconvenience.