Add an overload for set_row_shown.

Hiding one row at a time in the game_load dialog, when the filter
resulted in an empty set was slow. Every time a row was hidden the next
row was selected and its callback called. Directly after that the newly
selected row was hidden as well... The speedup is quite noticable.

changelog

@@ -10,6 +10,7 @@ Version 1.7.10+svn:
    * Allow a gui2 timer delete itself in its callback
    * Fix various bugs when a unit has more MP than its maximum
    * Fix not redrawing a grid when set to hidden
+   * Add helper functions to show/hide rows in a listbox
 
 Version 1.7.10-1.8beta3:
  * Campaigns:

src/gui/widgets/listbox.cpp

@@ -119,6 +119,37 @@ void tlistbox::set_row_shown(const unsigned row, const bool shown)
 	}
 }
 
+void tlistbox::set_row_shown(const std::vector<bool>& shown)
+{
+	assert(generator_);
+	assert(shown.size() == get_item_count());
+
+	twindow *window = get_window();
+	assert(window);
+
+	const int selected_row = get_selected_row();
+
+	bool resize_needed;
+	{
+		twindow::tinvalidate_layout_blocker invalidate_layout_blocker(*window);
+
+		for(size_t i = 0; i < shown.size(); ++i) {
+			generator_->set_item_shown(i, shown[i]);
+		}
+		resize_needed = !content_resize_request();
+	}
+
+	if(resize_needed) {
+		window->invalidate_layout();
+	} else {
+		set_dirty();
+	}
+
+	if(selected_row != get_selected_row() && callback_value_changed_) {
+		callback_value_changed_(this);
+	}
+}
+
 const tgrid* tlistbox::get_row_grid(const unsigned row) const
 {
 	assert(generator_);

src/gui/widgets/listbox.hpp

@@ -107,6 +107,20 @@ public:
 	 */
 	void set_row_shown(const unsigned row, const bool shown);
 
+	/**
+	 * Makes a row visible or invisible.
+	 *
+	 * Use this version if you want to show hide multiple items since it's
+	 * optimized for that purpose, for one it calls the selection changed
+	 * callback only once instead of serveral times.
+	 *
+	 * @param shown               A vector with the show hide status for every
+	 *                            row. The number of items in the vector must
+	 *                            be equal to the number of items in the
+	 *                            listbox.
+	 */
+	void set_row_shown(const std::vector<bool>& shown);
+
 	/**
 	 * Returns the grid of the wanted row.
 	 *