[BUGFIX] RTE: Tab key in Chrome inserts weird SPAN tags

Problem: The problem arises when indent/outdent buttons are not
configured in toolbar. Default WebKit action inserts span tag with
special class.

Solution: Stop the event and avoid the default action: do nothing.

Fixes: #47274
Releases: 4.5, 4.7, 6.0, 6.1
Change-Id: I973080f4fd8107b2cf1e2bebea905600dc8ca2e1
Reviewed-on: https://review.typo3.org/20059
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland