Ignore F12 as a terminal key to be able to easily open the dev tools.

Instead of having to use a right click and the 'inspect element' menu item.
Fix container size computation.

Of course, the char-size needs to taken from the PRE element rendering the
terminal, not from the body/container element.
Fix browserscreen resizing.

Track the number of total lines in the browserscreen to be able to compute
the line-origin offset when resizing the screen. We must compute this value
on the server as the resize may also change the current cursor position.
Render the cursor.