[CHART] Fix typo in comment.
[TREELIST] Fix doxygen comment.

 * Escape special characters '<' and '>'.
 * Improve wording.
[misc] mc_font_size() now takes one more parameter, ...

... a flag specifying whether font external leading should be incorporated
into the calculated font size.

Also adapted use of the function.
[TABLE, GRID] Big rework, including its public interface.

 * Simplify the interface by removing some redundancies and special cases.
 * Do not interpret column[0] as headers for rows, nor row[0] as headers
   for columns. Instead allocated special buffers for the headers.
 * MC_HTABLE can hold normal string data in all cells including
   the headers as well as the values (MC_HVALUE).
 * Rework the ex_grid.c exampple accordingly.