Nested menu structure

[kkoreilly]

We need to decide how to specify nested menus when using the new automatic, comment-directive-based approach for toolbars and menubars.

Proposal 1: Children specify their parents

For example:

//gi:menubar -parent File
func (me *Me) Save() error
//gi:menubar -parent File
func (me *Me) SaveAs(filename gi.FileName) error
//gi:menubar -parent File
func (me *Me) Open(filename gi.FileName) error

If the parent exists (for example, if there was an actionable File button), they are added to that parent. If the parent does not exist, it creates an artificial parent, a File button that doesn't do anything but contain the children.
You can also have nested parents:

//gi:menubar -parent File/Export -label PNG
func (me *Me) ExportPNG(filename gi.FileName) error
//gi:menubar -parent File/Export -label JPG
func (me *Me) ExportJPG(filename gi.FileName) error

If you have nested parents, it will follow the same rules as above; it will create as few artificial parents as possible (so if there is an actionable File/Export button, it will use that), but it will create as many artificial parents as needed (like os.MkdirAll). For example, in the export example above, assuming there were no other functions, it would create an artificial File menu button, and then it would create an artificial Export button inside of the File menu, and then it would create the real buttons inside of that

Pros:

• You do not have to make the artificial parents yourself
• It is clear where each button goes
• Nesting is easy; you just have to add a /

Cons:

• You do not have a list of everything in each sub-menu group in the same place, so it could be hard to maintain
• It is impossible to specify properties (the icon, tooltip, shortcut, etc) on artificial parents

Proposal 2: Parents specify their children

For example:

//gi:menubar -container -sub-menus Save,SaveAs,Open
func (me *Me) File()
//gi:menubar
func (me *Me) Save() error
//gi:menubar
func (me *Me) SaveAs(filename gi.FileName) error
//gi:menubar
func (me *Me) Open(filename gi.FileName) error

For every sub-menu group you want, you must declare a container function. If this container function is not actionable itself, then it must be marked with -container. You then list out the submenus on the container function.
This also supports nested parents:

//gi:menubar -container -sub-menus Export
func (me *Me) File()
//gi:menubar -container -sub-menus ExportPNG,ExportJPG
func (me *Me) Export()
//gi:menubar -label PNG
func (me *Me) ExportPNG(filename gi.FileName) error
//gi:menubar -label JPG
func (me *Me) ExportJPG(filename gi.FileName) error

You must create each container parent yourself.

Pros:

• It is explicit what menu items are in each sub-menu group; you don't have to hunt them all down
• It is easy to specify properties on containers

Cons:

• It requires manually defining each container, which is tedious and confuses people using your type in actual code, as you have a bunch of methods that don't do anything.
• It is difficult to make nested submenus, as you have to add a container for each one

[kkoreilly]

We are no longer using magic comment directives for menus, pursuant to Principle 1: Code is always the best solution.