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
[PDI-11236] "Preview data" panel handle value metas incorrectly
PDI-10942 - Job get emails JobEntry does not mark emails as 'read' when load emails content.

Added fix. Remote email folder is always attempt to open in RW mode if
protocol is not POP3, attempt to read email content marks email as read.
Junit tests added.
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.
Merge pull request #289 from stepanovdg/PDI-11236

[PDI-11236] "Preview data" panel handle value metas incorrectly
approved and merged
Merge pull request #215 from prakapenka/PDI-10942

PDI-10942 - Job get emails JobEntry does not mark emails as 'read' when load emails content