Completely cleanup the folders management (create/subscribe etc) code.

Completely cleanup the folders management (create/subscribe etc) code.
- Remove the many small src/folders_*, concentrate their functionality
  in functions/folder_manip.php and call that from folders.php.
- Saves duplicate code and an IMAP call here and there.
- Improve the code itself, replace complex structures with simpler ones
  which acheive the same.
- Improve UI by adding Cancel buttons where appropriate.
- Some other small fixes.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@9140 7612ce4b-ef26-0410-bec9-ea0150e637f0