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/main.cpp

@@ -92,6 +92,7 @@ int main(int argc, char** argv)
     window->set_has_alpha_channel(true);
     window->set_main_widget(&terminal);
     window->move_to(300, 300);
+    terminal.apply_size_increments_to_window(*window);
     window->show();
 
     auto menubar = make<GMenuBar>();