I open up a windowmanager and add a textfield and listbox:
editor.windowManager.open({ title: \'Insert caption\', body: [ {type: \'textbox\',
I found out while searching in the official TinyMCE plugins. So this is how it's done:
{type: 'listbox', name: 'align', label: 'align', 'values': [ {text: 'Left', value: 'left'}, {text: 'Right', value: 'right'}, {text: 'Center', value: 'center'} ] }