Support backspace, linefeed and carriage return in text mode

[lanceewing]

When AGI is in text mode, and a message is displayed that has backspace, linefeed, or carriage return in it, then those characters should behave like you'd expect them to in a console, i.e. backspace deletes the previous character, carriage return goes back to the start of the current line, and line feed goes down to the next line. The original AGI interpreter did this, but currently AGILE does not.

I don't know which, if any, original Sierra games used these characters in messages. This was discovered by editing a game in WinAGI and then testing on AGILE and the original interpreter.