Allow the screen to be moved to a different monitor in a reverse direction

[ErbaZZ]

Originally, the nextscreen binding can only move a screen to the next monitor in one direction only. For people with 3 or more monitors, this can be an annoyance when a screen is moved past their "intended" monitor, since they need to go through all the other monitors again.

This tiny PR adds a new "previousscreen" binding to support the moving of a screen in a reverse direction.

Changes:

• Remove the hardcoded animation duration on _moveNextScreenStep
• Rename _moveNextScreenStep function to _moveScreenStep and add a direction argument
• Add previousscreen key binding
• Add shortcut and description to README.md