Update itext_key_behavior.mixin.js

Added behaviour for keys pagUP ,pagDOWN , HOME, END, TAB and ESC. ( mimic meta key, that on windows i don't know where it is )
Removed rendering of squares in firefox.

There are two moveCursorLeft, two moveCursorRight, two moveCursorUp, two moveCursorDown.
You defined functions for moving the cursor for the arrow keys, and then with the help of CMD button ( metakey ) you use the same functions to go to: start of text, end of text, start of line, end of line.
I bind home, end, pgUp and pgDown keys to those functions ( not having a cmd button on my laptop ) and i obtain the same functions.