gtk: handle ctrl+return to toggle fullscreen

This is a follow-up to #172 and adds fullscreen handling to the GTK
apprt.

Works reliably for the current window and mimics exactly what happens
when F11 is pressed, which seems to be the standard keybinding for GTK
apps to toggle fullscreen.

src/apprt/gtk.zig

@@ -459,6 +459,16 @@ const Window = struct {
         }
     }
 
+    /// Toggle fullscreen for this window.
+    fn toggleFullscreen(self: *Window) void {
+        const is_fullscreen = c.gtk_window_is_fullscreen(self.window);
+        if (is_fullscreen == 0) {
+            c.gtk_window_fullscreen(self.window);
+        } else {
+            c.gtk_window_unfullscreen(self.window);
+        }
+    }
+
     /// Grabs focus on the currently selected tab.
     fn focusCurrentTab(self: *Window) void {
         const page_idx = c.gtk_notebook_get_current_page(self.notebook);
@@ -795,6 +805,10 @@ pub const Surface = struct {
         c.gtk_widget_show(alert);
     }
 
+    pub fn toggleFullscreen(self: *Surface) void {
+        self.window.toggleFullscreen();
+    }
+
     pub fn newTab(self: *Surface) !void {
         try self.window.newTab();
     }