Improve the FileDownload widget

[maximlt]

Fixes #1299

The intent of this PR is to fix various bugs with the FileDownload widget. I had to clearly define the behaviour of the widget, that to me follows three cases:

1. embed=True --> The widget is a link that looks like a button (right click to "Save as..." is possible)
2. auto=False --> There are two steps, first the widget is a button, after it's clicked it's a link (right click possible)
3. auto=True --> The widget is a button (right click to "Save as..." isn't possible).

To get there I had to make the following changes:

• On the python side: the biggest change is the new _transfers model attribute. It took me a long time before I dared to touch the python side but I found out it was the only solution that worked and allowed to fix all the bugs. When the widget is a button, a click will increment _clicks in typescript which will be reflected in python and trigger _transfer(). This method will increment _transfers, this signal will be caught on the typescript side, where a link will be created and clicked to download the data.
• On the typescript side: I've opted for an approach that is in favour of rendering first the widget and then updating it (that is not completely true as disabling/enabling the widget actually renders it again because of an inherited callback). It's quite a change compared to the first implementation, I believe it was required to cover all the cases supposedly offered by the widget.

Example fixed:

Updating button type, disabled, filename fixed:

[codecov]

Codecov Report

Merging #1306 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1306      +/-   ##
==========================================
+ Coverage   87.77%   87.81%   +0.04%     
==========================================
  Files         124      124              
  Lines       13485    13509      +24     
==========================================
+ Hits        11836    11863      +27     
+ Misses       1649     1646       -3     

Impacted Files	Coverage Δ
panel/models/widgets.py	100.00% <100.00%> (ø)
panel/tests/widgets/test_misc.py	98.11% <100.00%> (+0.38%)	⬆️
panel/widgets/misc.py	93.08% <100.00%> (+2.17%)	⬆️

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[philippjfr]

Thanks for this, not an easy fix to make after all. The approach makes a lot of sense to me though so just let me know if you think it's ready.

[maximlt]

Relieved that you're OK with all those changes!

I've seen some typos and things to be improved in the docs (the second sentence isn't complete here). I'll fix that and then it's ready.

[maximlt]

I think it's ready now.

[philippjfr]

Looks ready to me, thanks so much for contributing!

[MarcSkovMadsen]

So great to see this merged. Just had feedback from one of my users that experienced this bug.