Bugfix, code reorganization and clean-up

* Bugfix for issue#16
* Replaced SignatureLcMarkStr and SignatureUcMarkStr with SignatureMarkOrder
  to avoid ambiguity when a line contains both a local and a global mark
* Corrected marker navigation hotkeys for next and previous marker to ]- and [-
* While creating mappings check that RHS is unassigned to avoid collisions with
  user-created mappings.
* While creating mappings check that LHS is unmapped to avoid re-assigning maps.
* Reorganized code to have a cleaner separation of the SignatureToggle and
  SignatureRefreshDisplay functions