libsmartcols: new scols_column_{refer,get}_annotation routines

libsmartcols: new scols_column_{refer,get}_annotation routines

The new routines can be used to add an annotation to columns
which are intended to serve as descriptions for column headers.

These annotations are formatted with OSC8 escape sequences and
then sent to the output stream. A supporting terminal can then
display the formatted string as a tooltip.

This can be fairly useful when an application wants to provide
a translated description for a column header name, since the
latter is sometimes a mere abbreviation and not necessarily
telling enough.

Addresses: #1291
Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>