GUI2/Modal Dialog: added a convenience set_retval wrapper

This wraps window::set_retval and acts as a companion to get_retval

src/gui/dialogs/modal_dialog.cpp

@@ -118,6 +118,13 @@ bool modal_dialog::show(const unsigned auto_close_time)
 	return retval_ == retval::OK;
 }
 
+void modal_dialog::set_retval(int retval)
+{
+	if(window_) {
+		window_->set_retval(retval);
+	}
+}
+
 field_bool* modal_dialog::register_bool(
 		const std::string& id,
 		const bool mandatory,

src/gui/dialogs/modal_dialog.hpp

@@ -170,7 +170,6 @@ class modal_dialog
 	 */
 	bool show(const unsigned auto_close_time = 0);
 
-
 	/***** ***** ***** setters / getters for members ***** ****** *****/
 
 	/** Returns a pointer to the dialog's window. Will be null if it hasn't been built yet. */
@@ -179,11 +178,15 @@ class modal_dialog
 		return window_.get();
 	}
 
+	/** Returns the cached window exit code. */
 	int get_retval() const
 	{
 		return retval_;
 	}
 
+	/** Convenience wrapper to set the window's exit code. */
+	void set_retval(int retval);
+
 	void set_always_save_fields(const bool always_save_fields)
 	{
 		always_save_fields_ = always_save_fields;
@@ -330,7 +333,16 @@ class modal_dialog
 	std::unique_ptr<window> window_;
 
 private:
-	/** Returns the window exit status, 0 means not shown. */
+	/**
+	 * The window's exit code (return value).
+	 *
+	 * We keep a copy here so it may be accessed even after the dialog is closed and
+	 * the window object is destroyed.
+	 *
+	 * This value is initially set to 0 (retval::NONE) meaning the dialog was not
+	 * shown. After @ref show returns, it will hold the most recent retval of the
+	 * window object, including any modifications made in @ref post_show.
+	 */
 	int retval_;
 
 	/**