[API][!!!] Fix API compatibility break in tslib_pibase

Revision 50cbf264 removed the old tslib_pibase constructor without
deprecating it first as required by TYPO3 core rules. This breaks
every FE plugin, which overrides the old tslib_pibase constructor.
The change was commited to Git here: https://review.typo3.org/1797

The fix: reintroduce the old constructor and deprecate it.

Change-Id: I66653025b9bf9e2d1e4a0dddb6d0913c50c387c2
Resolves: #28208
Releases: 4.6
Reviewed-on: http://review.typo3.org/3325
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert