Add GridSpec layout [doc-build]

[philippjfr]

Implements a GridSpec layout based on the new GridBox model in bokeh. It works quite differently from the previous approach, you no longer have to declare the number of cols and rows, you simply declare a size (or sizing_mode='stretch_both') and then start filling the grid:

Supersedes #31

[jbednar]

Seems very cool! We should have a demo example using this approach, with some numeric readouts and other things that aren't plots, as in the various dashboards you find if you search for "dashboard" images.

[philippjfr]

Here's a more complex example:

and here's an error when trying to assign to a region of the grid that's already occupied:

IndexError: Specified region overlaps with the following existing object(s) in the grid:

    (0, 0): HTML(str, background='#FF0000')

    (1, 0): HTML(str, background='#00FF00')

    (1, 1): VTK(str, camera={'position': [-21.49009035...})

The following shows a view of the grid (empty: 0, occupied: 1, overlapping: 2):

[[2 2 2]
 [2 2 2]
 [2 2 2]
 [1 1 0]
 [1 1 1]]

[jbednar]

Is there a simple way to center content in each grid cell's region, both vertically and horizontally? Presumably no one would want A, B, C, D to be in the upper left like that.

[philippjfr]

Is there a simple way to center content in each grid cell's region, both vertically and horizontally?

There is probably some combination of CSS that will do it, worth looking into but not tied up with this PR.

[codecov-io]

Codecov Report

Merging #338 into master will decrease coverage by 0.16%.
The diff coverage is 83.83%.

@@            Coverage Diff             @@
##           master     #338      +/-   ##
==========================================
- Coverage   87.05%   86.89%   -0.17%     
==========================================
  Files          88       88              
  Lines        7263     7540     +277     
==========================================
+ Hits         6323     6552     +229     
- Misses        940      988      +48

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
panel/__init__.py	100% <100%> (ø)	⬆️
panel/tests/test_layout.py	100% <100%> (ø)	⬆️
panel/pane/image.py	75.89% <30.76%> (-6.11%)	⬇️
panel/pane/markup.py	90.58% <50%> (-3.17%)	⬇️
panel/layout.py	89.35% <78.82%> (-7.06%)	⬇️

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