[BUGFIX] TSFE->altPageTitle can not be set in extensions

It is not possible to set $GLOBALS['TSFE']->altPageTitle
in USER_INT extensions anymore to modify the page title.

This has been possible before in the 4.x branches.

Fix this by implementing the title generation like all
other header data.

Resolves: #54467
Releases: 6.2, 6.1
Change-Id: I3d5be34b95cb295b92732d2f2f9bd1f123812365
Reviewed-on: https://review.typo3.org/27135
Tested-by: Alexander Opitz
Reviewed-by: Markus Klein
Tested-by: Markus Klein