1. Adding a separate MY_COLLATION_HANDLER
   my_collation_ucs2_general_mysql500_ci_handler
   implementing a proper order for ucs2_general_mysql500_ci
   The problem happened because ucs2_general_mysql500_ci
   erroneously used my_collation_ucs2_general_ci_handler.

2. Cosmetic changes: Renaming:
   - plane00_mysql500 to my_unicase_mysql500_page00
   - my_unicase_pages_mysql500 to my_unicase_mysql500_pages
   to use the same naming style with:
   - my_unicase_default_page00
   - my_unicase_defaul_pages

3. Moving code fragments from
   - handler::check_collation_compatibility() in handler.cc
   - upgrade_collation() in table.cc
   into new methods in class Charset, to reuse the code easier.