Enable callees based on implementors search for JDT.UI by letting the
configurability to JDT.LS.

This fix also improve the callee hierarchy by adding support for showing
multiple implementations and expand on those. Also this fix add support
for triggering call hierarchies on method declarations on interface
methods and abstract methods.

Change-Id: I5eb1a309fdce1d93ff5486fce6bad046d5d091c1
Signed-off-by: Gayan Perera <gayanper@gmail.com>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/189117
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Tested-by: Jeff Johnston <jjohnstn@redhat.com>
Reviewed-by: Jeff Johnston <jjohnstn@redhat.com>