extending selection in list and tree without use of keyboard

[Vlad Harchev <hvv hippo ru>]

 Hello,

 Currently it's possible to extend selection to the non-continuous set of
rows in list and tree by holding keyboard modifiers.
 It seems to me that it would be handy to provide ability to extend selection
to the non-continuous set of rows in list and tree without using keyboard. I
propose to use middle mouse button for this - i.e. if one clicks with middle
button and drags a pointer over rows, the subject rows are added to the
selection even if the selection becomes non-continuos.

 Seems to be easy to implement.

 What do you guys think about this?

 Best regards,
  -Vlad