GModelSelection: Add size() and a const overload of for_each_index()

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/1cb661ae690

Libraries/LibGUI/GModelSelection.h

@@ -12,6 +12,7 @@ public:
     {
     }
 
+    int size() const { return m_indexes.size(); }
     bool is_empty() const { return m_indexes.is_empty(); }
     bool contains(const GModelIndex& index) const { return m_indexes.contains(index); }
     bool contains_row(int row) const
@@ -36,6 +37,13 @@ public:
             callback(index);
     }
 
+    template<typename Callback>
+    void for_each_index(Callback callback) const
+    {
+        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
     {