adds a button to the toolbar
| id | string|number | id of a button |
| pos | number | position of a button |
| text | string | text for a button (null for no text) |
| imgEn | string | image for the enabled state (null for no image) |
| imgDis | string | image for the disabled state (null for no image) |
myToolbar.addButton(id, pos, text, imgEn, imgDis);