#604 Add support for Mouse back-button to the 7-zip File Manager.

[Tommi Prami]

Add support for Mouse back-button to the 7-zip File Manager. If used to use back button of mouse in Windows Explorer etc, it would make the app way faster to use.

Need to handle WM_XBUTTONUP message and wparam XBUTTON1 (1 as decimal).
Mouse Back button can call same code as the button pointed in the attachment.

Forward button would be nice bonus, then wparam is XBUTTON2 (2 as decimal if not mistaken)