gcombobox: constructor for a combobox
In gWidgets2: Rewrite of gWidgets API for Simplified GUI Construction

A combobox can be either a drop down list (editable=FALSE), or a drop-down list and edit area (a combobox).

Non exported helper function to coerce items into a data frame. First column contains the values, second stock icons, third tooltips

Ensure that value is a data frame. One can pass a vector or a one-column data frame to inidicate the possible values for selection, a second column is used for an icons (if possible), a third for a tooltip (if possible).

Change handler for a non-editabe combobox is called when a new value is selected. For editable comboboxes, the handler is also called when the text entry box is activated.

The svalue method for a combobox object refers to its main property, the selected value. When index=FALSE (or NULL) the value is returned. If index=TRUE the index of the object within the set of items is used.

gcombobox(
  items,
  selected = 1,
  editable = FALSE,
  coerce.with = NULL,
  handler = NULL,
  action = NULL,
  container = NULL,
  ...,
  toolkit = guiToolkit()
)

.gcombobox(
  toolkit,
  items,
  selected = 1,
  editable = FALSE,
  coerce.with = NULL,
  handler = NULL,
  action = NULL,
  container = NULL,
  ...
)

gdroplist(...)

.make_gcombobox_items(value)

## S3 replacement method for class 'GComboBox'
 x[i , j, ...] <- value

## S3 method for class 'GComboBox'
addHandlerChanged(obj, handler, action = NULL, ...)

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

`items`	Items to select from. A vector or a data frame. If a data frame, then first column is values. Second is optional, but can specify a stock icon name, third is optional and can be used to specify a tooltip. These may not be supported in all toolkits.
`selected`	integer. Which item (by index) is selected. Use -1 for no selection
`editable`	logical. Is user allowed to edit value
`coerce.with`	A function of function name to be called before selected value is returned by `svalue`
`handler`	Called when combobox value is changed.
`action`	passed to handler
`container`	parent container
`...`	passed to parent container's `add` method
`toolkit`	toolkit
`value`	new items for selection
`x`	combobox object
`i`	item index
`j`	ignored
`obj`	object receiving event and emitting a signal to the handler
`index`	NULL or logical. If `TRUE` and widget supports it an index, instead of a value will be returned.
`drop`	NULL or logical. If widget supports it, drop will work as it does in a data frame or perhaps someother means.

Returns an object of class GComboBox for which the following methods are overriden:

svalue Return selected value by name or (if index=TRUE by index). The latter only if editable=FALSE.
svalue<- Set the selected value by value or if index=TRUE by index.
[ return items to select from
[<- Set items to select from.