Keytests: Make the wrapper sendKeystring() around sendKeystringLocal() be recursive

Keytests: Make the wrapper sendKeystring() around sendKeystringLocal() be recursive

This makes the wrapper working smoothly even for more complicated
input lines.
Also we do not need the window- parameter in call to xvkbd.
Still, there _is_ something fishy. At first run it can happen that
some keytest fail. But not reproducible at subsequent calls.
It feels like QT would cache some data and therefore lyx reacts
later faster.
This work is a result of collaboration with Tommaso Cucinotta.