Implement mono_text item

Since we don't need to do shaping or layouting for latin1 text, skip it.
MonoText does this. However, to do this correctly the textdecoder needs
to be moved into the parser.
Add mono_text

and signal to monotext if the current line only contains latin1 chars so
that we can skip layouting alltogether. Else do layout but still cut
corners so that it is significantly faster than QQuickText