This introduces new settings for all column-based views to make them
configurable via tigrc. The configuration format is a simple
space-separated list of strings, each containing comma-separated
key=value pairs. This allows to enable columns such as line numbers
individually for each view (as mentioned in issue #222) and hiding the
ID column in the blame view.
Due to the difference in configuration format all old view UI display
settings are now obsolete with no graceful upgrade. However, warnings
for all obsolete settings are emitted pointing to the view column types.