[FEATURE] Allow escaping of dots in TypoScript key

This patch allows the use of backslashes to escape dots in
TypoScript keys, for example:

my\.escaped\.key = test

will result in a single key "my.escaped.key" with the
value "test".

Additionally it is possible to protect backslashes from beeing
interpreted as escape characters by using double backslashes.

Resolves: #29461
Documentation: #56249
Releases: 6.2
Change-Id: Idcaae75d9a8800134f4a82e0740ddabe35b3df89
Reviewed-on: https://review.typo3.org/19480
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader
Reviewed-by: Stefan Neufeind
Reviewed-by: Alexander Stehlik
Tested-by: Alexander Stehlik
Reviewed-by: Markus Klein
Tested-by: Markus Klein