Support middle mouse paste on Linux

Adds for pasting the selection clipboard on middle mouse button on Linux.

Pastes on `mouseup` instead of `mousedown` which seemed to be consistent with of native inputs in Firefox and Chromium as well as Sublime on Ubuntu 14.04.

Copies all selection changes instead of just mouse driven selection changes so that things like `ctrl-D` and `ctrl-L` go to the selection clipboard.

Closes #2244
Closes #2272
Closes #3027