Switch DatetimePicker start/end to param.Date

[philippjfr]

A DatetimePicker should allow datetimes for the start and end values.

[hoxbro]

Just for reference see #3107 why it was not done in the first place.

[philippjfr]

Thanks @hoxbro! Given that in most cases people just use datetime even when they really want a date I think I'm still going to allow this but document the limitation more clearly.

[codecov]

Codecov Report

Merging #3202 (2ab676b) into master (c796c02) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 2ab676b differs from pull request most recent head 89b7546. Consider uploading reports for the commit 89b7546 to get more accurate results

@@           Coverage Diff           @@
##           master    #3202   +/-   ##
=======================================
  Coverage   83.15%   83.15%           
=======================================
  Files         193      193           
  Lines       25695    25695           
=======================================
  Hits        21367    21367           
  Misses       4328     4328           

Impacted Files	Coverage Δ
panel/widgets/input.py	96.33% <100.00%> (ø)

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