TreeModel: More API cleanup

- introduce topLevelItemCount (similar to QTreeWidget)
- squash untypedTreeLevelItems()
- rename removeItems() to clear()
- rename removeItem() to takeItem()
- rename treeLevelItems<> to itemsAtLevel<>

Change-Id: I0f1bb4110f7687b20da3d92e3d943858645a9fa2
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/debugger/breakhandler.cpp

@@ -401,7 +401,7 @@ void BreakHandler::deletionHelper(BreakpointModelId id)
 {
     Breakpoint b = breakpointById(id);
     QTC_ASSERT(b, return);
-    removeItem(b.b);
+    takeItem(b.b);
     delete b.b;
 }
 
@@ -1132,7 +1132,7 @@ void BreakHandler::saveSessionData()
 
 void BreakHandler::loadSessionData()
 {
-    removeItems();
+    clear();
     loadBreakpoints();
 }
 
@@ -1216,7 +1216,7 @@ void BreakHandler::changeLineNumberFromMarkerHelper(BreakpointModelId id)
     Breakpoint b = breakpointById(id);
     QTC_ASSERT(b, return);
     BreakpointParameters params = b.parameters();
-    removeItem(b.b);
+    takeItem(b.b);
     delete b.b;
     appendBreakpoint(params);
 }