ProcessManager: Start working on a graphical process manager.

I need a table view widget for this thing, so I'm also using this to
prototype a model/view thingy.

LibGUI/GModelIndex.h

@@ -0,0 +1,19 @@
+#pragma once
+
+class GModelIndex {
+public:
+    GModelIndex() { }
+    GModelIndex(int row, int column)
+        : m_row(row)
+        , m_column(column)
+    {
+    }
+
+    bool is_valid() const { return m_row != -1 && m_column != -1; }
+    int row() const { return m_row; }
+    int column() const { return m_column; }
+
+private:
+    int m_row { -1 };
+    int m_column { -1 };
+};

LibGUI/GTableModel.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <AK/AKString.h>
+#include <LibGUI/GModelIndex.h>
+
+class GTableModel {
+public:
+    GTableModel() { }
+    virtual ~GTableModel() { }
+
+    virtual int row_count() const = 0;
+    virtual int column_count() const = 0;
+    virtual String row_name(int) const { return { }; }
+    virtual String column_name(int) const { return { }; }
+    virtual int column_width(int) const { return 0; }
+    virtual String data(int row, int column) const = 0;
+    virtual void set_selected_index(GModelIndex) { }
+    virtual GModelIndex selected_index() const { return GModelIndex(); }
+    virtual void update() = 0;
+};