Toggle fullscreen on super/ctrl+return, only macOS for now

This fixes or at least is the first step towards #171:

- it adds `cmd/super + return` as the default keybinding to toggle
  fullscreen for currently focused window.
- it adds a keybinding handler to the embedded apprt and then changes
  the macOS app to handle the keybinding by toggling currently focused
  window.

src/Surface.zig

@@ -1135,6 +1135,12 @@ pub fn keyCallback(
                     } else log.warn("runtime doesn't implement gotoSplit", .{});
                 },
 
+                .toggle_fullscreen => {
+                    if (@hasDecl(apprt.Surface, "toggleFullscreen")) {
+                        self.rt_surface.toggleFullscreen();
+                    } else log.warn("runtime doesn't implement toggleFullscreen", .{});
+                },
+
                 .close_surface => self.close(),
 
                 .close_window => {

src/apprt/embedded.zig

@@ -64,6 +64,9 @@ pub const App = struct {
 
         /// Goto tab
         goto_tab: ?*const fn (SurfaceUD, usize) callconv(.C) void = null,
+
+        /// Toggle fullscreen for current window.
+        toggle_fullscreen: ?*const fn (SurfaceUD) callconv(.C) void = null,
     };
 
     core_app: *CoreApp,
@@ -371,6 +374,15 @@ pub const Surface = struct {
         func(self.opts.userdata, n);
     }
 
+    pub fn toggleFullscreen(self: *Surface) void {
+        const func = self.app.opts.toggle_fullscreen orelse {
+            log.info("runtime embedder does not toggle_fullscreen", .{});
+            return;
+        };
+
+        func(self.opts.userdata);
+    }
+
     /// The cursor position from the host directly is in screen coordinates but
     /// all our interface works in pixels.
     fn cursorPosToPixels(self: *const Surface, pos: apprt.CursorPos) !apprt.CursorPos {

src/config.zig

@@ -441,6 +441,13 @@ pub const Config = struct {
             }
         }
 
+        // Toggle fullscreen
+        try result.keybind.set.put(
+            alloc,
+            .{ .key = .enter, .mods = ctrlOrSuper(.{}) },
+            .{ .toggle_fullscreen = {} },
+        );
+
         // Mac-specific keyboard bindings.
         if (comptime builtin.target.isDarwin()) {
             try result.keybind.set.put(

src/input/Binding.zig

@@ -207,6 +207,9 @@ pub const Action = union(enum) {
     /// Close the window, regardless of how many tabs or splits there may be.
     close_window: void,
 
+    /// Toggle fullscreen mode of window.
+    toggle_fullscreen: void,
+
     /// Quit ghostty
     quit: void,