Implement Clear and Reset actions with hotkeys

[jeremypw]

Fixes #181 (?)

I am unsure whether the Clear action should also clear the scrollback buffer. At the moment it does not because the "Reset" action does that. The native hotkey <Control>L does not clear the buffer but typing clear into the terminal does.

Is it worth implementing two actions - "Clear Screen Only" and "Clear Screen and Scrollback"?

The XFCE terminal labels the reset action "Clear Scrollback and Reset" - should we follow or is that too verbose?

Should the "Clear Screen" menu option be labelled "Clear Screen Only"?

[zeebok]

Works as advertised. To me this resolves what they were asking for, though it does make me curious what was handling the CTRL + L shortcut before this?

[jeremypw]

Lets merge and refine with further PRs if required.