Code cleanup

[juliusHuelsmann]

• clean up the code conveyed in the vim patch
  • keep changes modular / in a different location from the st code for maintainability
  • make sure that 'public' functions can be called with arbitrary arguments and do not fail (e.g. check against inputs that currently cannot occur, but could occur if the function were used in a different was)
  • try to break down large (new) chunks of code into small ones (functions / files).
  • check the new code with static analysers
  • adhere to the code style used in the proj

[juliusHuelsmann]

New patch dev was moved to separate repo, the current version of the patch is not applied on the master branch, but on the new default branch for release 0.8.3.