The class ExtensionManager is actually a utility
to manage extensions. Rename it to better reflect
it's purpose.

Renaming in submodules will be done seperately.
The third step will then be to remove the old
namespaced class name from the class alias map
and raise the submodule pointers

Related: #42661
Releases: 6.0

Change-Id: I0be22f117e9e4121001b6a14118d289834438066
Reviewed-on: http://review.typo3.org/16163
Reviewed-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel