Location component

[philippjfr]

Massive redesign of the original Location component written by @MarcSkovMadsen. The problem is that it's not really a Viewable component and therefore should not be a widget. It's also weird adding it to a layout if it doesn't actually display anything. Therefore I introduced non-viewable baseclasses for Panel components and the location is automatically added as a Document root and then made accessible via pn.state.location.

In order to make the Location component work without also being a Viewable objects the baseclasses are now organized as such:

• Layoutable(Parameterized): Defines parameters concerned with layout and style
• ServableMixin(): Mixin class that defines methods to serve object on a server
• Renderable(Parameterized): Defines methods that allow the object to rendered to a bokeh model
• Viewable(Renderable, Layoutable, ServableMixin): Defines methods that make the object viewable in a notebook, save it and serve it
• Syncable(Renderable): Defines methods that allow parameters to be synced with bokeh model properties
• Reactive(Viewable, Syncable): A syncable viewable that defines various methods to link it to other objects.

Supersedes #1101

[philippjfr]

Was hoping to get this into 0.9.0 after all but it touches too many internals for me to safely include it.

[jbednar]

Looks good to me. I wonder if the panel/viewable.py changes ought to be in a separate PR that explains the class hierarchy changes on their own, to be merged before this one.

[codecov]

Codecov Report

Merging #1150 into master will increase coverage by 0.06%.
The diff coverage is 88.84%.

@@            Coverage Diff             @@
##           master    #1150      +/-   ##
==========================================
+ Coverage   87.53%   87.59%   +0.06%     
==========================================
  Files         117      124       +7     
  Lines       12952    13192     +240     
==========================================
+ Hits        11337    11555     +218     
- Misses       1615     1637      +22     

Impacted Files	Coverage Δ
panel/io/state.py	80.32% <46.15%> (-9.87%)	⬇️
panel/io/notebook.py	58.64% <50.00%> (-0.48%)	⬇️
panel/io/server.py	37.65% <60.00%> (ø)
panel/viewable.py	68.69% <80.48%> (-8.69%)	⬇️
panel/util.py	85.94% <85.29%> (-0.15%)	⬇️
panel/reactive.py	85.40% <85.40%> (ø)
panel/io/location.py	96.77% <96.77%> (ø)
panel/io/embed.py	85.99% <100.00%> (ø)
panel/layout/base.py	93.47% <100.00%> (ø)
panel/layout/spacer.py	74.07% <100.00%> (ø)
... and 21 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dcfa4c6...ce93bf0. Read the comment docs.