If true, multiple values may be selected.
The SelectItem will either render as a drop-down allowing multiple
selections, or a multi-row list of options similar to a small headerless
com.smartgwt.client.widgets.grid.ListGrid, based on the
com.smartgwt.client.types.MultipleAppearance setting.
The logical value of the formItem, as retrieved by
com.smartgwt.client.widgets.form.fields.FormItem#getValue and set via
com.smartgwt.client.widgets.form.fields.FormItem#setValue, is an Array of
Strings reflecting the selected values.
Note: multiple:true
SelectItems do not currently support
optionDataSource binding. You can get around this by calling
com.smartgwt.client.data.DataSource#fetchData directly and calling
com.smartgwt.client.data.List#getValueMap to obtain a valueMap.