Don't show useless fold icon in structure view (KT-17254, KT-21200)

#KT-21200 In Progress
 #KT-17254 Fixed

idea/idea-analysis/src/org/jetbrains/kotlin/idea/structureView/KotlinStructureViewModel.kt

@@ -16,15 +16,20 @@
 
 package org.jetbrains.kotlin.idea.structureView
 
+import com.intellij.ide.structureView.StructureViewModel
 import com.intellij.ide.structureView.StructureViewModelBase
+import com.intellij.ide.structureView.StructureViewTreeElement
 import com.intellij.ide.util.treeView.smartTree.*
 import com.intellij.openapi.editor.Editor
 import com.intellij.util.PlatformIcons
+import org.jetbrains.kotlin.psi.KtClassOrObject
 import org.jetbrains.kotlin.psi.KtDeclaration
+import org.jetbrains.kotlin.psi.KtEnumEntry
 import org.jetbrains.kotlin.psi.KtFile
 
-class KotlinStructureViewModel(ktFile: KtFile, editor: Editor?) : StructureViewModelBase(ktFile, editor, KotlinStructureViewElement(ktFile, false)) {
-
+class KotlinStructureViewModel(ktFile: KtFile, editor: Editor?) :
+        StructureViewModelBase(ktFile, editor, KotlinStructureViewElement(ktFile, false)),
+        StructureViewModel.ElementInfoProvider {
     init {
         withSuitableClasses(KtDeclaration::class.java)
         withSorters(Sorter.ALPHA_SORTER)
@@ -34,6 +39,16 @@ class KotlinStructureViewModel(ktFile: KtFile, editor: Editor?) : StructureViewM
 
     override fun getFilters() = FILTERS
 
+    override fun isAlwaysShowsPlus(element: StructureViewTreeElement): Boolean {
+        val value = element.value
+        return (value is KtClassOrObject && value !is KtEnumEntry) || value is KtFile
+    }
+
+    override fun isAlwaysLeaf(element: StructureViewTreeElement): Boolean {
+        // Local declarations can in any other declaration
+        return false
+    }
+
     companion object {
         private val NODE_PROVIDERS = listOf(KotlinInheritedMembersNodeProvider())
         private val FILTERS = arrayOf<Filter>(PublicElementsFilter)