Fix a severe performance regression on filtering add-ons

For some reason, it only occurred in debug builds when I tested.

The problem was that partial relayout didn't respect
invalidate_layout_blocker. As a result, in situations such as populating a
listbox one entry at a time, the layout system ended up performing dozens
of full relayouts that, not surprisingly, is extremely expensive.

Regression from commit 01035f0.

src/gui/widgets/grid.cpp

@@ -385,6 +385,11 @@ void grid::demand_reduce_height(const unsigned /*maximum_height*/)
 
 void grid::request_placement(dispatcher&, const event::ui_event, bool& handled, bool&)
 {
+	if (get_window()->invalidate_layout_blocked()) {
+		handled = true;
+		return;
+	}
+
 	point size = get_size();
 	point best_size = calculate_best_size();
 	if(size.x >= best_size.x && size.y >= best_size.y) {

src/gui/widgets/window.hpp

@@ -256,6 +256,12 @@ class window : public panel, public cursor::setter
 		window& window_;
 	};
 
+	/** Is invalidate_layout blocked, see invalidate_layout_blocker. */
+	bool invalidate_layout_blocked() const
+	{
+		return invalidate_layout_blocked_;
+	}
+
 	/**
 	 * Updates the size of the window.
 	 *
@@ -521,7 +527,7 @@ class window : public panel, public cursor::setter
 	/** The variables of the canvas. */
 	wfl::map_formula_callable variables_;
 
-	/** Is invalidate layout blocked see invalidate_layout_blocker. */
+	/** Is invalidate_layout blocked, see invalidate_layout_blocker. */
 	bool invalidate_layout_blocked_;
 
 	/** Avoid drawing the window.  */