Add playback controls to dock menu on MacOS

ui/app_delegate_darwin.m

@@ -4,6 +4,28 @@
 
 extern void appReopened(void);
 extern void appShouldTerminate(void);
+extern void dockMenuItemClicked(int index);
+
+// Generic action target for Dock menu items - the menu contents (titles,
+// separators, count) are fully defined from the Go side. Each item's tag
+// is the index of its Go-side callback, dispatched through the single
+// dockMenuItemClicked export.
+@interface DockMenuTarget : NSObject
+- (void)itemClicked:(id)sender;
+@end
+
+@implementation DockMenuTarget
+- (void)itemClicked:(id)sender {
+    dockMenuItemClicked((int)[(NSMenuItem *)sender tag]);
+}
+@end
+
+// dockMenuBuilding is assembled by dockMenuBegin/AddItem/AddSeparator and
+// published to dockMenu (returned from -applicationDockMenu:) by
+// dockMenuCommit, so an in-progress rebuild is never seen half-built.
+static DockMenuTarget *dockMenuTarget;
+static NSMenu *dockMenuBuilding;
+static NSMenu *dockMenu;
 
 @interface ReopenDelegate : NSObject<NSApplicationDelegate>
 @property (nonatomic, strong) id<NSApplicationDelegate> wrapped;
@@ -34,6 +56,10 @@ extern void appShouldTerminate(void);
     return YES;
 }
 
+- (NSMenu *)applicationDockMenu:(NSApplication *)sender {
+    return dockMenu;
+}
+
 - (BOOL)respondsToSelector:(SEL)sel {
     return [super respondsToSelector:sel] ||
            [self.wrapped respondsToSelector:sel];
@@ -50,3 +76,28 @@ void installReopenDelegate(void) {
     d.wrapped = [NSApp delegate];
     [NSApp setDelegate:d];
 }
+
+void dockMenuBegin(void) {
+    if (!dockMenuTarget) {
+        dockMenuTarget = [DockMenuTarget new];
+    }
+    dockMenuBuilding = [[NSMenu alloc] initWithTitle:@""];
+}
+
+void dockMenuAddItem(const char *title, int index) {
+    NSMenuItem *item = [[NSMenuItem alloc] initWithTitle:[NSString stringWithUTF8String:title]
+                                                   action:@selector(itemClicked:)
+                                            keyEquivalent:@""];
+    item.target = dockMenuTarget;
+    item.tag = index;
+    [dockMenuBuilding addItem:item];
+}
+
+void dockMenuAddSeparator(void) {
+    [dockMenuBuilding addItem:[NSMenuItem separatorItem]];
+}
+
+void dockMenuCommit(void) {
+    dockMenu = dockMenuBuilding;
+    dockMenuBuilding = nil;
+}