Arbitrary base converter and calculator

[stefnotch]

• [x] Convert between arbitrary bases >= 2
• [x] Support converting rational numbers like 3A.7F
• [x] Do mathematics in another base
  • [x] +
  • [x] -
  • [x] *
  • [x] /
  • [ ] Support rational numbers like 3A.7F: Out of scope
• [x] Have a proper user interface

[stefnotch]

Eventual fixes and improvements can be done in future PRs, I merged this so that we don't run into merge conflicts. Am about to have fun.