Add support for Legend item fill policies

[mattpap]

This adds rudimentary support for item fill policies:

import numpy as np

from bokeh.plotting import figure, show

x = np.linspace(0, 4*np.pi, 50)
y = np.sin(x)

TOOLS = "pan,wheel_zoom,box_zoom,reset,save,box_select"

p1 = figure(title="Legend Example", tools=TOOLS, width=300, height=300)

p1.circle(x, 1*y, legend_label="1*sin(x)", fill_color=None, line_color="green")

p1.square(x, 2*y, legend_label="2*sin(x)", fill_color=None, line_color="orange")
p1.line(x, 2*y, legend_label="2*sin(x)", line_color="orange")

p1.circle(x, 3*y, legend_label="3*sin(x)", fill_color=None, line_color="blue")

p1.square(x, 4*y, legend_label="4*sin(x)", fill_color=None, line_color="tomato")
p1.line(x, 4*y, legend_label="4*sin(x)", line_color="tomato")

p1.circle(x, 5*y, legend_label="5*sin(x)", fill_color=None, line_color="purple")

p1.square(x, 6*y, legend_label="6*sin(x)", fill_color=None, line_color="pink")
p1.line(x, 6*y, legend_label="6*sin(x)", line_color="pink")

p1.legend.title = 'Markers'
p1.legend.ncols = 2
p1.legend.orientation = "vertical"
p1.legend.click_policy = "mute"
p1.legend.title_location = "above"
p1.legend.item_background_policy = "even"
show(p1)

fixes #12692

/cc @mosc9575

[codecov]

Codecov Report

Merging #12701 (481787d) into branch-3.1 (f63969a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           branch-3.1   #12701   +/-   ##
===========================================
  Coverage       92.23%   92.23%           
===========================================
  Files             314      314           
  Lines           19682    19687    +5     
===========================================
+ Hits            18154    18159    +5     
  Misses           1528     1528           

[mosc9575]

This looks great. Thank you @mattpap.

[bryevdv]

The first two comments are suggestions/advisory but I do want to get discussion and agreement regarding any changes to defaults.

[bryevdv]

@mattpap I think this PR is very close, would you like me to finish off the three suggestions above so that it can be merged while you focus on other PRs?

[mattpap]

Examples' test failure is unrelated and fixed in PR #12754.