[BUGFIX] RTE.default.buttons.blockstyle not configurable

Problem: Data from the TextStyle combobox,derived from the textstyle
configuration, appears in the BlockStyle combobox.
Solution:
1. Load the global classes configuration from PageTSconfig only once.
2. Force the initialization of the parsed classes object in the
HTMLArea.CSS.Parser class constructor. (ExtJS bug?)

Releases: master, 6.2
Resolves: #58034
Change-Id: Iaf2f8b7ee8cca347f5cd08619677965fdd1a0168
Reviewed-on: http://review.typo3.org/33914
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>