improvements in ObxActions. Refs: #67.
- stepSize is now a variable and can be set by the user.
- a step can compute multiple primes depending on the size of stapSize
- the output text is opened at the beginning of the calculation
- an aux view is used for the results
- feedback for starting and finishing the calculation has been added

Signed-off-by: Josef Templ <josef.templ@gmail.com>