Adding event to the middle mouse button click.

This is a continuation of https://github.com/modxcms/revolution/pull/449 in another realization. 
And prevent changes in the current window
Add open in new window action to middle mouse button click in trees

Merge remote-tracking branch 'origin/pr/458' into develop

* origin/pr/458:
  And prevent changes in the current window
  Adding event to the middle mouse button click.

Conflicts:
	manager/assets/modext/widgets/core/tree/modx.tree.js