FileManager+LibGUI: Fix two folder-related crashes (#569)

Fix a crash when opening a folder, and another one when trying to open
a newly created folder.

It was not safe to modify a GModelSelection while it's being iterated over.

Fixes #536.

Libraries/LibGUI/GFileSystemModel.cpp

@@ -31,6 +31,15 @@ struct GFileSystemModel::Node {
         ASSERT_NOT_REACHED();
     }
 
+    void cleanup()
+    {
+        for (auto& child: children) {
+            child->cleanup();
+            delete child;
+        }
+        children.clear();
+    }
+
     void traverse_if_needed(const GFileSystemModel& model)
     {
         if (type != Node::Directory || has_traversed)
@@ -146,15 +155,22 @@ GFileSystemModel::~GFileSystemModel()
 
 void GFileSystemModel::update()
 {
-    // FIXME: Support refreshing the model!
-    if (m_root)
-        return;
+    cleanup();
 
     m_root = new Node;
     m_root->name = m_root_path;
     m_root->reify_if_needed(*this);
 }
 
+void GFileSystemModel::cleanup()
+{
+    if (m_root) {
+        m_root->cleanup();
+        delete m_root;
+        m_root = nullptr;
+    }
+}
+
 int GFileSystemModel::row_count(const GModelIndex& index) const
 {
     if (!index.is_valid())

Libraries/LibGUI/GFileSystemModel.h

@@ -37,6 +37,7 @@ private:
 
     struct Node;
     Node* m_root { nullptr };
+    void cleanup();
 
     GIcon m_open_folder_icon;
     GIcon m_closed_folder_icon;

Libraries/LibGUI/GModelSelection.h

@@ -33,17 +33,27 @@ public:
     template<typename Callback>
     void for_each_index(Callback callback)
     {
-        for (auto& index : m_indexes)
+        for (auto& index : indexes())
             callback(index);
     }
 
     template<typename Callback>
     void for_each_index(Callback callback) const
     {
-        for (auto& index : m_indexes)
+        for (auto& index : indexes())
             callback(index);
     }
 
+    Vector<GModelIndex> indexes() const
+    {
+        Vector<GModelIndex> selected_indexes;
+
+        for (auto& index : m_indexes)
+            selected_indexes.append(index);
+            
+        return selected_indexes;
+    }
+
     // FIXME: This doesn't guarantee that what you get is the lowest or "first" index selected..
     GModelIndex first() const
     {