Make LineEditor::get_line() responsible for printing the prompt. That way
we can re-prompt after clearing the screen on ^L.
This makes the Serenity Terminal feel a little bit more like home :^)
When resizing the terminal, we now clear the entire current line and reset
the shell's LineEditor input state. This makes it look and feel kinda the
same as xterm.
Fixes#286.
Hook this up in Terminal so that the '\a' character generates a beep.
Finally emit an '\a' character in the shell line editing code when
backspacing at the start of the line.
