gmenu: menu constructor, main and popup
In jverzani/gWidgets2: Rewrite of gWidgets API for Simplified GUI Construction

A menu may be viewed as a heirarchical collection of buttons, each invoked by clicking on the button. These buttons are exposed through submenus. More generally, a widget may replace the button. This widget intends to support buttons (gactions), separators (gseparator), radio button (gradio) and checkbutton (gcheckbox), but this may be toolkit independent. When using a radio button or checkbox, one should pass in a parent argument to the constructor – not a container.

generic for toolkit dispatch

add menubar items to a menu

For a menubar, svalue returns the list of action items etc. that defined the menubar. This can be useful to access the underlying item being proxied. (For gaction items the enabled<- method may be used on the item, but this may not extend to gradio and gcheckbox items)

for a menubar, svalue<- replaces the menubar items with new ones specified by value.

gmenu(menu.list, popup = FALSE, container = NULL, ...,
  toolkit = guiToolkit())

.gmenu(toolkit, menu.list = list(), popup = FALSE, container = NULL, ...)

## S3 method for class 'GMenuBar'
add(obj, child, expand = FALSE, fill = NULL,
  anchor = NULL, ...)

## S3 method for class 'GMenuBar'
svalue(obj, index = NULL, drop = NULL, ...)

## S3 replacement method for class 'GMenuBar'
 svalue(obj, index=NULL, ...) <- value

`menu.list`	A list defining the menu structure. Named sub lists determine the submenu titles and structure. The list may have components of class: `GAction`, mapped to a button; `GSeparator`, mapped to a horizontal separator; `GRadio`, mapped to linked buttons; or `GCheckbox`, mapped to a checkbox button.
`popup`	logical. If true, make a popup window to be added through a handler call
`container`	A parent container. When a widget is created it can be incorporated into the widget heirarchy by passing in a parent container at construction time. (For some toolkits this is not optional, e.g. gWidgets2tcltk or gWidgets2WWW2.)
`...`	These values are passed to the `add` method of the parent container. Examples of values are `expand`, `fill`, and `anchor`, although they're not always supported by a given widget. For more details see add. Occasionally the variable arguments feature has been used to sneak in hidden arguments to toolkit implementations. For example, when using a widget as a menubar object one can specify a parent argument to pass in parent information, similar to how the argument is used with gaction and the dialogs.
`toolkit`	Each widget constructor is passed in the toolkit it will use. This is typically done using the default, which will lookup the toolkit through `guiToolkit`.
`obj`	parent object
`child`	list. a menubar list or gmenu instance.
`expand`	NULL or logical. For box containers controls whether a child will expand to fill the allocated space.
`fill`	NULL or character. For box containers. The value of `fill` (not always respected) is used to control if expansion happens vertically (`y`), horizontally (`x`) or both (`both` or `TRUE`). For vertically filled box containers, children always fill horizontally (atleast) and for horizontally filled box containers, children always fill vertically (atleast). This is important to realize when trying to size buttons, say.
`anchor`	NULL or integer. For box containers. The anchor argument is used to position the child within the parent when there is more space allocated than the child requests. This is specified with a Cartesian pair in -1,0,1 x -1, 0, 1.
`index`	ignored
`drop`	ignored
`value`	a list or menu bar specifying the new men ubar