demo: support up to 128 logic channels, enable the lower 8 by default

demo: support up to 128 logic channels, enable the lower 8 by default

Bump the number of supported logic channels from 8 to 128. This is
mostly motivated to test the 64 channels limit which some of the
components/subprojects of the sigrok project might have (input/output
modules, user interfaces).

Only automatically enable the first 8 of the 128 total logic channels,
i.e. default to the previous behaviour. Prepare to only enable part of
the set of analog channels, but stick with their being active by default
as well.

Factor out the choice for the default logic pattern, too. This allows
for easier adjustment of the default configuration, when settings are
concentrated in a single spot.