Add toggle to highlight ambiguities in sequences

[westram]

Goal: toggle between normal sequence display and a layout which highlights ambiguity codes

• add a GUI toggle and provide a hotkey (e.g. ctrl-q)
• add a configuration window to define
  • foreground color for N, other ambiguity codes and standard bases (using same syntax as 'sequence colors')
  • background color used for ambiguity codes (e.g. one of the search highlight colors)

Highlighting ambiguity codes

• shall override all other color settings (by default)
• optionally show standard background colors for non-ambiguous characters (e.g. search-results, marked color, …)

[westram]

Milestone renamed