Internally it was only invoke callback set for an
operator template. This invoke was setting such
properties as mouse_x and mouse_y and was calling
an exec function.
This meant that t seemed to be really easy to
use node.select operator from by giving a mouse
positions, but in fact it wasn't possible (because
it requires exec callback)
This commit sets operator's template exec callback,
which makes it possible using node.select from
python.
791be2f79f