Deprecate FileUtil and move the code to FileUtils

Deprecate FileUtil and move the code to FileUtils

As discussed on https://git.eclipse.org/r/53836 it does not make sense
to have two similar utility classes in same package with intersecting
functionality. To not break the API, all methods from FileUtil are
copied to FileUtils, all FileUtil API is made deprecated and redirecting
now to FileUtils. Moved simple methods which are available in Java 7 API
are made package private and can be removed at any point later entirely
(right now they are in use).

Bug: 475070
Change-Id: Idffcf9840496c448173af7c052d8898ada68e27b
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>