Add concept of size increments to windowing system.

Use this to implement incremental resizing for Terminal so that we only
ever resize to fit a perfect number of rows and columns.

This is very nice. :^)

Applications/Terminal/Terminal.h

@@ -21,6 +21,8 @@ public:
     void flush_dirty_lines();
     void force_repaint();
 
+    void apply_size_increments_to_window(GWindow&);
+
 private:
     virtual void event(GEvent&) override;
     virtual void paint_event(GPaintEvent&) override;