The main advantages of not clearing and rebuilding the whole list is
that it doesn't loose the folding and selection (as far as the selected
row(s) still exist after the update, of course), and it reduces
flickering upon update.

The current implementation works in 3-steps:
1) mark all rows as invalid;
2) insert/update the rows, according to the new tag list, marking them
   as valid;
3) remove all rows that are still invalid.
This walks (rows) the first time, (tags*rows) the second and (rows) the
third. This also uses an extra column to store the row's validity.

A (probably) better implementation would be to:
1) walk the current rows, updating them if necessary, or removing them;
2) add the remaining tags that weren't there before.
This is probably faster in theory (and probably also in practice), but
it needs to refactor a lot the code to easily update *or* create a row,
what the current code does not provide.
Basically this is would be a two-pass update, walking (rows*tags) in
the first pass, and only the remaining non-added tags in the second.


git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5562 ea778897-0a13-0410-b9d1-a72fbfd435f5