[v3] Add option for showing the toolbar in fullscreen mode on macOS (#3282)

* Add option for showing the toolbar in fullscreen mode on macOS

* Add an example demonstrating the ShowToolbarWhenFullscreen option

* Update docs

* Add changelog entry

* Run go mod tidy

mkdocs-website/docs/en/API/fullapi.md

@@ -1760,6 +1760,8 @@ type MacTitleBar struct {
     UseToolbar bool
     // HideToolbarSeparator will hide the toolbar separator
     HideToolbarSeparator bool
+    // ShowToolbarWhenFullscreen will keep the toolbar visible when the window is in fullscreen mode
+	ShowToolbarWhenFullscreen bool
     // ToolbarStyle is the style of toolbar to use
     ToolbarStyle MacToolbarStyle
 }