LibGUI: Add GModelSelection to help implementing multiple-select views

Each GAbstractView now has a GModelSelection backed by a simple
HashTable<GModelIndex>. When the selection changes somehow, the view
gets notified via the notify_selection_changed() callback.

In the future it will probably make sense to move to using some kind of
ranges as the internal representation instead.

Libraries/LibGUI/GModelSelection.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <AK/HashTable.h>
+#include <LibGUI/GModelIndex.h>
+
+class GAbstractView;
+
+class GModelSelection {
+public:
+    GModelSelection(GAbstractView& view)
+        : m_view(view)
+    {
+    }
+
+    bool is_empty() const { return m_indexes.is_empty(); }
+    bool contains(const GModelIndex& index) const { return m_indexes.contains(index); }
+
+    void set(const GModelIndex&);
+    void add(const GModelIndex&);
+    bool remove(const GModelIndex&);
+    void clear();
+
+    template<typename Callback>
+    void for_each_index(Callback callback)
+    {
+        for (auto& index : m_indexes)
+            callback(index);
+    }
+
+    // FIXME: This doesn't guarantee that what you get is the lowest or "first" index selected..
+    GModelIndex first() const
+    {
+        if (m_indexes.is_empty())
+            return {};
+        return *m_indexes.begin();
+    }
+
+private:
+    GAbstractView& m_view;
+    HashTable<GModelIndex> m_indexes;
+};