Avoid issue where active content views are inappropriately destroyed

FIX: Fix a bug that corrupts the editor's internal view tree data structure on some
types of edits, putting the editor in a broken state.

Closes codemirror/dev#1306

src/blockview.ts

@@ -29,7 +29,7 @@ export class LineView extends ContentView implements BlockView {
       if (!this.dom) source.transferDOM(this) // Reuse source.dom when appropriate
     }
     if (hasStart) this.setDeco(source ? source.attrs : null)
-    mergeChildrenInto(this, from, to, source ? source.children : [], openStart, openEnd)
+    mergeChildrenInto(this, from, to, source ? source.children.slice() : [], openStart, openEnd)
     return true
   }
 

src/contentview.ts

@@ -247,7 +247,7 @@ export abstract class ContentView {
   getSide() { return 0 }
 
   destroy() {
-    for (let child of this.children) child.destroy()
+    for (let child of this.children) if (child.parent == this) child.destroy()
     this.parent = null
   }
 }

src/inlineview.ts

@@ -106,7 +106,7 @@ export class MarkView extends ContentView {
     if (source && (!(source instanceof MarkView && source.mark.eq(this.mark)) ||
                    (from && openStart <= 0) || (to < this.length && openEnd <= 0)))
       return false
-    mergeChildrenInto(this, from, to, source ? source.children : [], openStart - 1, openEnd - 1)
+    mergeChildrenInto(this, from, to, source ? source.children.slice() : [], openStart - 1, openEnd - 1)
     this.markDirty()
     return true
   }