More (small) marker and label sizes

[talmo]

• [ ] Add a couple more default marker sizes
• [ ] Make the available marker sizes configurable via the preferences yaml
  • [ ] Make sure to add validation and fallback to a default set of sizes since this is user configurable

Discussed in https://github.com/talmolab/sleap/discussions/1854

^{Originally posted by **olinesn** July 8, 2024} Hello, I'm writing to ask if an additional marker and font size options could be added? Most of the larger options are not very useful, and the steps at the low end are too big. Maybe add "9" between 6 and 12 for fonts, and a "3" in marker sizes between 2 and 4? Thanks! Stefan