review refactor: CtModifiable add/removeModifier

[surli]

Fix the documentation for CtModifiable#addModifier (Fix #1616)
Change the signature of CtModifiable#removeModifier to be consistent

[monperrus]

could you add a test on the returned type / object of add/removeModifier?

[surli]

could you add a test on the returned type / object of add/removeModifier?

Yes, in fact I was thinking about creating an enforcer rule for the set/get/add/remove methods of the different interfaces. But in another PR.

[surli]

could you add a test on the returned type / object of add/removeModifier?

Done.

[spoon-bot]

Detected changes by Revapi: 1.

Old API: fr.inria.gforge.spoon:spoon-core:jar:6.0.0-20171109.234516-155

New API: fr.inria.gforge.spoon:spoon-core:jar:6.0.0-SNAPSHOT

Name	Change 1
Old	method CtModifiable::removeModifier(ModifierKind)
New	method CtModifiable::removeModifier(ModifierKind)
Code	java.method.returnTypeChanged
Description	The return type changed from 'boolean' to 'T extends spoon.reflect.declaration.CtModifiable'.
Breaking	binary: breaking
Name	Change 1
:---:	:---:
Old	method CtModifiable::removeModifier(ModifierKind)
New	method CtModifiable::removeModifier(ModifierKind)
Code	java.generics.elementNowParameterized
Description	Element now defines formal type parameters.
Breaking	binary: non_breaking,
Name	Change 1
:---:	:---:
Old	method CtModifiable::removeModifier(ModifierKind)
New	method CtModifiable::removeModifier(ModifierKind)
Code	java.generics.formalTypeParameterAdded
Description	A new formal type parameter added to element: 'T extends spoon.reflect.declaration.CtModifiable'.
Breaking	binary: non_breaking

[ghost]

There appears to be a large amount of code duplication here that could be eliminated using a common superclass.