Made the Slider control ignore calls to its SetValue method while it's being dragged.
semcomedi: improve messages
semcomedi: fix find_best_oversampling_rate so that OSR should always work

we were trying too hard to find a common multiplier between two compatible
read/write periods, instead of just picking a bigger OSR.
This caused some write periods to be incompatible with every OSR.

Added a test case to reproduce this problem.
gui: unsubscribe/subscribe to dataflow when the dwell time is changed

Changing the dwell time of the HardwareComponent doesn't cancel the current
acquisition. If the dwell time is large, it will cause a long acquisition which
is running even if it was a mistake by the user.

Reduce the user's surprise by automatically cancelling acquisitions when the
dwell time is big.
Merge branch 'master' of vpn.delmic.com:odemis