feat: CtImportVisitor

[pvojtechovsky]

Simplifies work with different kinds of imports -> CtImportVisitor

[spoon-bot]

API changes: 6 (Detected by Revapi)

Old API: fr.inria.gforge.spoon:spoon-core:jar:7.2.0-20181101.234717-66 / New API: fr.inria.gforge.spoon:spoon-core:jar:7.2.0-SNAPSHOT

	Method was added to an interface.
Old	none
New	method CtImport#accept(CtImportVisitor)
Breaking	binary: non_breaking

	Method was added to an interface.
Old	none
New	method CoreFactory#createTypeMemberWildcardImportReference()
Breaking	binary: non_breaking

	Method was added to an interface.
Old	none
New	method Factory#createTypeMemberWildcardImportReference(CtTypeReference)
Breaking	binary: non_breaking

	Method was removed.
Old	method CoreFactory#createWildcardStaticTypeMemberReference()
New	none
Breaking	binary: breaking,

	Method was removed.
Old	method Factory#createWildcardStaticTypeMemberReference(CtTypeReference)
New	none
Breaking	binary: breaking,

	Method was added to an interface.
Old	none
New	method CtVisitor#visitCtTypeMemberWildcardImportReference(CtTypeMemberWildcardImportReference)
Breaking	binary: non_breaking

[pvojtechovsky]

It is ready for review.

[monperrus]

LGTM.