review feat: Add helpers methods in CtModifiable

[surli]

fix #1657

[spoon-bot]

Detected changes by Revapi: 6.

Old API: fr.inria.gforge.spoon:spoon-core:jar:6.0.0-20171114.234550-171

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

Name	Change 1
Old	none
New	method CtModifiable::isAbstract()
Code	java.method.addedToInterface
Description	Method was added to an interface.
Breaking	binary: non_breaking,

Name	Change 2
Old	none
New	method CtModifiable::isFinal()
Code	java.method.addedToInterface
Description	Method was added to an interface.
Breaking	binary: non_breaking,

Name	Change 3
Old	none
New	method CtModifiable::isPrivate()
Code	java.method.addedToInterface
Description	Method was added to an interface.
Breaking	binary: non_breaking,

Name	Change 4
Old	none
New	method CtModifiable::isProtected()
Code	java.method.addedToInterface
Description	Method was added to an interface.
Breaking	binary: non_breaking,

Name	Change 5
Old	none
New	method CtModifiable::isPublic()
Code	java.method.addedToInterface
Description	Method was added to an interface.
Breaking	binary: non_breaking,

Name	Change 6
Old	none
New	method CtModifiable::isStatic()
Code	java.method.addedToInterface
Description	Method was added to an interface.
Breaking	binary: non_breaking,