PDI-10140 - "retrieve the...first mails" option doesn't work (for POP3 protocol)

What was done:
1) implemented get first N emails for POP3 via batch
PDI-10140 - "retrieve the...first mails" option doesn't work (for POP3 protocol)

What was done:
1) Fixed UI for the step (initial value was not displayed)
2) Added variables support
PDI-10140 - "retrieve the...first mails" option doesn't work (for POP3 protocol)

What was done:
1) added handling of the initializing when no data was input
2) added unit test
3) fixed issue with batch processing i.e. batch from 1 to 4 returned 4
mails and 1 to 5 returned also 4 mails
Merge pull request #278 from kolinus/PDI-10140-2

Pdi 10140 - "retrieve the...first mails" option doesn't work (for POP3 protocol)
Approved and merged.