Drop "$oldway" mailbox list generation code from left_main. The new code

Drop "$oldway" mailbox list generation code from left_main. The new code
has been surviving fine since 1.3.0 and as far as we know there are no
current problems with it. What this patch does:
- Remove a lot of obsolete code from left_main.php.
- Remove configurable tweak 'oldway' from config (not renumbering conf.pl
  menu at this stage to avoid confusion of menu item numbers).
- Coding style fixes to mailbox generation code.
- Remove unneccessary (only one cell) HTML tables from left_main.
- Add support for unselectable folder color (color[15]) which was missing
  from ListBoxes.
Note: advancedtree is still a configurable tweak as it's not as mature
and has dhtml-dependencies.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@9217 7612ce4b-ef26-0410-bec9-ea0150e637f0