buf_writeln is now changing the color context, if needed, before writing
to a window instead of writing in standard color and then re-coloring
every line. This is a significant efficiency gain in one of the most
used functions.
Implemented function to highlight a line in a buffer in a chosen color
pair. The navigator is now using it instead of asj_hl_line to visualize
the cursor position and return deselected lines to their previous color.
Re-Implemented color pair definitions that allows for algorithmic color
selection such that, for example, lines can be highlighted in a
different background color while maintaining their foreground color.