Usage of deprecated jetbrains Api

[sambaranhazra]

Current stable build (1.1.1) of intellij-swagger has some deprecated usage as specified by the jetbrains report, this needs to be resolved to keep the plug-in up to date.

Deprecated methods usages (5)
AnnotationHolder.createWeakWarningAnnotation(...) (2)
Deprecated method AnnotationHolder.createWeakWarningAnnotation(...) is invoked in UnusedRefAnnotator.warn(...)
Deprecated method AnnotationHolder.createWeakWarningAnnotation(...) is invoked in UnusedRefAnnotator.warn(...)
ResourceUtil.getResource(Class, String, String) (2)
Deprecated method ResourceUtil.getResource(Class, String, String) is invoked in YamlSchemaInspection.createVisitor(...)
Deprecated method ResourceUtil.getResource(Class, String, String) is invoked in JsonSchemaInspection.createVisitor(...)
IconLoader.getIcon(String) (1)
Deprecated method IconLoader.getIcon(String) is invoked in Icons.<clinit>()
Deprecated classes usages (4)
HashSet (4)
Deprecated class HashSet is referenced in SpecIndexer.getReferencedFilesYaml(...)
Deprecated class HashSet is referenced in SpecIndexer.getReferencedFilesJson(...)
Deprecated class HashSet is referenced in OpenApiFileIndex$1.read(DataInput)
Deprecated class HashSet is referenced in SwaggerFileIndex$1.read(DataInput)```

[asaarilahti]

All remaining usages of deprecated IntelliJ APIs should have been removed in #341 .