Fix ambigious ownership for slices passed to setTitle and copyToClipboard

src/vxfw/App.zig

···
             .set_mouse_shape => |shape| self.vx.setMouseShape(shape),
             .request_focus => |widget| self.wants_focus = widget,
             .copy_to_clipboard => |content| {
+                defer self.allocator.free(content);
                 self.vx.copyToSystemClipboard(self.tty.writer(), content, self.allocator) catch |err| {
                     switch (err) {
                         error.OutOfMemory => return Allocator.Error.OutOfMemory,
···
                 };
             },
             .set_title => |title| {
+                defer self.allocator.free(title);
                 self.vx.setTitle(self.tty.writer(), title) catch |err| {
                     std.log.err("set_title error: {}", .{err});
                 };

src/vxfw/vxfw.zig

···
         try self.addCmd(.{ .request_focus = widget });
     }
 
+    /// Copy content to clipboard.
+    /// content is duplicated using self.alloc. 
+    /// Caller retains ownership of their copy of content.
     pub fn copyToClipboard(self: *EventContext, content: []const u8) Allocator.Error!void {
-        try self.addCmd(.{ .copy_to_clipboard = content });
-    }
+        try self.addCmd(.{ .copy_to_clipboard = try self.alloc.dupe(u8, content) });
+    } 
 
+    /// Set window title.
+    /// title is duplicated using self.alloc.
+    /// Caller retains ownership of their copy of title.
     pub fn setTitle(self: *EventContext, title: []const u8) Allocator.Error!void {
-        try self.addCmd(.{ .set_title = title });
+        try self.addCmd(.{ .set_title = try self.alloc.dupe(u8, title) });
     }
 
     pub fn queueRefresh(self: *EventContext) Allocator.Error!void {