Add open in new window action to middle mouse button click in trees

Adding event to the middle mouse button click.

This is a continuation of https://github.com/modxcms/revolution/pull/449 in another realization. 
(cherry picked from commit 15adb2098840024853ef551eb58a97bd58604598)
(cherry picked from commit 94a1a769b339d894d80c78115917f427489552ad)

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