Bunch of papercuts.

- Alt+Backspace now acts as delete.
- Alt+Left-click now acts as right click.
- Clicking Share no longer clears selection.
- Middle click does nothing (no longer acts like primary click)

Bug: 31991398
Bug: 31988739
Bug: 31991827
Bug: 31992756
Change-Id: Ic86a65bb2d1e94a8a7fb140bff624483632e84fb