Add explicit string_escape state to the Pyte escape sequence state machine.

Allows for a simpler, optimized feed function as we don't need to look back
in the stream to determine the meaning of the current character.
Change iframe mode API to be entirely ecma-48 string based.

Use different x-schirm-* headers to assign different meanings to messages.
Explicitly utf-8 encode text in iframe document mode.
Use new API in schirmclient and demo applications.