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 };
+};