Cursor can reach invalid positions

[Tossrock]

The text entry cursor can reach invalid positions pretty easily. Things that I've found triggering this:

• Backspacing beyond the start of the command then starts deleting the prompt. You can also backspace onto previous lines.
• Home can move the cursor to the beginning of the line, ie before command >
• Mouse select can enter invalid positions in multiple ways:
  • Single clicking in an invalid position on the same line is allowed (ie, within command >)
  • Single clicking from a valid cursor position to a position above the current line selects text from the start of the command (ie, directly after command >) up to the click position
  • Single clicking from an invalid cursor position can select arbitrary scrollback text
  • Double clicking a word selects it
  • etc

[marijnz]

I put in basic prevention for this, which should cover most of the cases you're describing. Let me know if there's still something fix-worthy, otherwise feel free to close this Issue.

Commit: https://github.com/marijnz/unity-shell/commit/3bc817e5baf871d30643a4c5666810f754fb0b0d

[Tossrock]

Tried it out, seems solid! Thanks for fixing this, makes the shell much more usable :)