More frame options in window functions

Attached is the fix pointed out in the previous CommitFest plus RANGE
offset support.

*fix
- move window regression test to another parallel group, but regarding
the limitation of 20 per group the union test goes to the group the
window test belonged to.
- allow NULL iswindowagg as an argument of AggGetMemoryContext
- change view name to longer one

*RANGE offset
- allow all of RANGE BETWEEN <value> PRECEDING/FOLLOWING AND <value>
PRECEDING/FOLLOWING for any data types that support ORDER BY and
additions/subtractions, which is extended design to the spec. The spec
says data types allowed in RANGE offset are only numeric and temporal
ones but we don't have such limitation.
- add "+"/"-" operator search code in parsing, which is used to
calculate frame bound, but I'm not sure if this is right approach.
- add more regression tests

Regards,

--
Hitoshi Harada