Support mouse side-button is a mapping.

[TylerHaoNhot]

Developer TODO (don't remove)

• create new branch. put issue number at start of name
• update documentation
• merge and delete branch (don't squash because want commit history to see why I made changes)

The default action of the side-button on the mouse is for back and forth the websites, but i want to change it to different mapping. When i tried to record the action, it's didn't work.