* Add ClearBuffer action
Clears all lines, both visible and those scrolled off the top of the viewport, making the prompt line the new first line and resetting the scrollbar thumb to the full height of the window.
This is the behavior that Hyper / xterm has for clearing the terminal.
* Combine ClearBuffer into ClearScrollback as enum with associated erase mode
Makes it easier to manage the different options of clearing the terminal.
