Added general support for linking Panel objects

[xavArtley]

Following PR #191
It focus only on the Links between widgets and Hv bokeh plots

I removed AxesLink which was a link between Hv bokeh plots
I removed dependencies to holoviews (duplicate code of the Link class of holoviews)
I will create another PR with some refactor of the links between Hv Plots to decorrelate Links from holoviews and links developed in Panel.

[codecov-io]

Codecov Report

Merging #199 into master will decrease coverage by 0.03%.
The diff coverage is 89.45%.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
- Coverage   90.12%   90.08%   -0.04%     
==========================================
  Files          31       33       +2     
  Lines        4942     5176     +234     
==========================================
+ Hits         4454     4663     +209     
- Misses        488      513      +25

Impacted Files	Coverage Δ
panel/__init__.py	52.17% <100%> (+1.06%)	⬆️
panel/widgets.py	95.82% <100%> (+0.02%)	⬆️
panel/viewable.py	70.4% <100%> (ø)	⬆️
panel/holoviews.py	84.18% <100%> (+0.94%)	⬆️
panel/links.py	83.68% <83.68%> (ø)
panel/tests/test_links.py	97.43% <97.43%> (ø)

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[xavArtley]

Now it is possible to link directly bokeh widgets or panel widgets to bokeh models

[philippjfr]

@xavArtley I've started generalizing the LinkCallback a bit so we can eventually apply arbitrary linking. Next I'll be adding a simpler API to specify widget links. Please let me know if you disagree with any of my changes so far.

[xavArtley]

[philippjfr]

@xavArtley I've once again refactored the code in order to make linking a completely general concept which can be applied both in Python and in JS. Have a look at the updated Links.ipynb to see what you think.

@jbednar When you get a minute I'd appreciate it if you could review and edit that notebook as well to see what you think.

[xavArtley]

@philippjfr Great work it's very intuitive.

[jbednar]

@philippjfr, that looks great! I've reworked that a bit:

• I changed the descriptions to clarify that there are three distinct ways to make links (.link, .param.watch, and .jslink); before the section structure made it seem like there were more.
• Added a hint at the top that we are building up to direct, pure HTML+JS links for plots (which is the most exciting reason to be reading this material)
• Changed the examples to work both line by line as before, but also as a web page where the entire notebook has been run at once. (Previously, starting at the top to read a Run-All-ed notebook would be very confusing, as the results from later cells not yet reached by the reader were affecting cells reached sooner by the reader.)
• Where possible, removed values initializing various widgets and instead forced a callback before the widgets were shown, to ensure that the values displayed were always consistent. (Before, changing a value then changing it back wouldn't necessarily result in the same display as at first; now it's always consistent where feasible.)
• Changed the last text box to a selector to make it simpler for users to change colors without encountering error conditions.

[jbednar]

@philippjfr, that looks great! I've reworked the Links.ipynb notebook a bit:

• I changed the descriptions to clarify that there are three distinct ways to make links (.link, .param.watch, and .jslink); before the section structure made it seem like there were more.
• Added a hint at the top that we are building up to direct, pure HTML+JS links for plots (which is the most exciting reason to be reading this material)
• Changed the examples to work both line by line as before, but also as a web page where the entire notebook has been run at once. (Previously, starting at the top to read a Run-All-ed notebook would be very confusing, as the results from later cells not yet reached by the reader were affecting cells reached sooner by the reader.)
• Where possible, removed values initializing various widgets and instead forced a callback before the widgets were shown, to ensure that the values displayed were always consistent. (Before, changing a value then changing it back wouldn't necessarily result in the same display as at first; now it's always consistent where feasible.)
• Changed the last text box to a selector to make it simpler for users to change colors without encountering error conditions.

[jbednar]

If those changes look ok to you, I'm happy to merge it!

[philippjfr]

@xavArtley If you're around could you apply the suggestions? Only the PR author can do it.

[philippjfr]

Thanks @xavArtley at working out the initial cut of this and thanks @jbednar for tidying up the docs.