Always exit through PresentationController.quit()

This ensures that the metadata will always be saved.

src/classes/metadata/pdf.vala

@@ -189,6 +189,13 @@ namespace pdfpc.Metadata {
             } 
         }
 
+        /*
+         * Called on quit
+         */
+        public void quit() {
+            this.save_to_disk();
+        }
+
         /**
          * Save the metadata to disk, if needed (i.e. if the user did something
          * with the notes or the skips)

src/classes/presentation_controller.vala

@@ -193,6 +193,11 @@ namespace pdfpc {
             this.fillActionNames();
         }
 
+        public void quit() {
+            this.metadata.quit();
+            Gtk.main_quit();
+        }
+
         public void set_overview(Window.Overview o) {
             this.overview = o;
         }
@@ -841,11 +846,6 @@ namespace pdfpc {
             }
         }
 
-        protected void quit() {
-            this.metadata.save_to_disk();
-            Gtk.main_quit();              
-        }
-
         /**
          * Parse the given time string to a Time object
          */

src/classes/window/presentation.vala

@@ -48,7 +48,7 @@ namespace pdfpc.Window {
             base( screen_num );
 
             this.destroy.connect( (source) => {
-                Gtk.main_quit();
+                presentation_controller.quit();
             } );
 
             this.presentation_controller = presentation_controller;

src/classes/window/presenter.vala

@@ -141,7 +141,7 @@ namespace pdfpc.Window {
             base( screen_num );
 
             this.destroy.connect( (source) => {
-                Gtk.main_quit();
+                presentation_controller.quit();
             } );
 
             this.presentation_controller = presentation_controller;