Support rebinding individual mouse buttons

[omentic]

Currently only the behaviour of left-click on various UI items is bindable. Right-click always opens the context menu, and middle-click does nothing. I would like to be able to bind middle-click with Halloy: specifically, I'd like it so that left-click on sidebar.buffer runs replace-pane, and middle-click runs new-pane.

The configuration options would probably have to be revamped to support this. There could be left_click / right_click / middle_click sub-dictionaries on either actions.buffer and actions.sidebar, or on each of their individual keys. This would also require exposing a context-menu configuration option if right_click is exposed.