| callback function on user item double-click selection (supplemental parameter are clicked item position, btn and mask)
position where nil is root element, 0::nil is first son of root element, 1::nil is second son of root element, etc..
btn can be any of following values :
| Value | Meaning |
| LBUTTON | left button is clicked |
| MBUTTON | middle button is clicked |
| RBUTTON | right button is clicked |
mask is a combination of values :
| Value | Meaning |
| MK_CONTROL | control key is pressed |
| MK_LBUTTON | left button is pushed |
| MK_MBUTTON | middle button is pushed |
| MK_RBUTTON | right button is pushed |
| MK_SHIFT | shift key is pushed |
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