This commit enables users to bind the mouse buttons (left, middle,
right buttons and the scroll wheel).
The default bindings now include the mouse bindings:
"MouseWheelUp": "ScrollUp",
"MouseWheelDown": "ScrollDown",
"MouseLeft": "MousePress",
"MouseMiddle": "PastePrimary",
Mouse buttons can now also be bound to normal actions. For example:
"MouseLeft": "Backspace"
This also means that plugins can access mouse event callbacks in the
standard way ('onAction').
More documentation for this will be coming soon.
Fixes#542