Various improvements to the DateFilter widget

[maximlt]

Includes various fixes to the DateFilter:

• On picker mode cast start/end/value to datetime.date
• Override the value Parameter by a CalendarDate or DateRange Parameters (let me know if there's a better way, that looks hacky!)
• Bidirectionally link the widget to the value Parameter, this allows URL query parameter changes to be synced back to the widget.

This is work in progress and requires other changes both in Param and Panel. I've also noticed accumulating calls in get_data on picker mode so there's something wrong there.

[codecov-commenter]

Codecov Report

Merging #276 (f92a7cb) into master (99a62e3) will decrease coverage by 0.19%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
- Coverage   65.42%   65.22%   -0.20%     
==========================================
  Files          57       57              
  Lines        5611     5628      +17     
==========================================
  Hits         3671     3671              
- Misses       1940     1957      +17     

Impacted Files	Coverage Δ
lumen/filters/base.py	63.07% <0.00%> (-2.70%)	⬇️
lumen/ui/variables.py	36.36% <0.00%> (-2.20%)	⬇️
lumen/ui/launcher.py	47.25% <0.00%> (-2.18%)	⬇️

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[maximlt]

This is ready for review @philippjfr

The accumulating calls to get_data I saw when picking a date were due to an issue in Param. The last commit just allows to link visible and disabled to the date widget, they were not linked which looks like an oversight.

A few other PRs were required to get this properly working:

• Don't try to get an Undefined bokeh property panel#3513
• DateRangeSlider: value and value_throttled turned to DateRange Parameters panel#3515 + DateRangeSlider: value_throttled is also a DateRange Parameter panel#3529
• Improve DateRange validation param#627
• Allow (de)serializing CalendarRange and DateRange Parameters param#625
• Support comparing date-like objects param#629

[maximlt]

@philippjfr I've reworked this PR quite a bit, c89cb89 is where you can see all the changes I've made.

Based on our discussion and the write-up in #293, I've implemented multi for the date filters, and I've added datetime filters. The implementation has changed quite a bit as instead of changing the value Parameter with self.param.add_parameter, I implemented __new__ to dispatch to classes that have override value with the right Parameter type.

There are a couple of decisions I've made which we'll need your approval:

• For multi=True and mode='picker', as there's no DateRangePicker available in Panel yet I've decided to fallback to a DatetimeRangePicker and emit a warning. This is the option 2 I mentioned in Meta-issue on date filters/widgets across Lumen, Panel and Param #293
• For multi=False and mode='slider', as there's no DatetimePicker I've decided to emit a warning and fall back to a DatetimePicker

It'd probably be worth:

• Adding some tests!

[philippjfr]

Adding tests separately. Thanks @maximlt!

[github-actions]

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