config: improve defaults handling

It is much better to initialize all configs to the same value and
to only set the ones which differ explicitly. This helps extending
them.
dmmcontrol: rework timeout and error handling

Less indention in the error flow is possible to improve readability
and the timeout can be used as function parameter with a default to
make it cleaner.
mainwindow: add display handling, fix rate setting

We want to be able to turn off the display during measurement. The
rate is also no trigger rate but a measurement rate instead.