Add support for formatters to ValueRef

[mattpap]

Example (side-by-side comparison):

import numpy as np

from bokeh.plotting import figure, show

N = 4000
x = np.random.random(size=N) * 100
y = np.random.random(size=N) * 100
radii = np.random.random(size=N) * 1.5
colors = np.array([(r, g, 150) for r, g in zip(50+2*x, 30+2*y)], dtype="uint8")

TOOLS="crosshair,pan,wheel_zoom,zoom_in,zoom_out,box_zoom,undo,redo,reset,tap,save,box_select,poly_select,lasso_select,examine,help"

p = figure(tools=TOOLS)

p.circle(x, y, radius=radii, fill_color=colors, fill_alpha=0.6, line_color=None)

from bokeh.models import HoverTool, Styles
from bokeh.models.dom import Div, ValueRef, Index, Template

style = Styles(
    display="grid",
    grid_template_columns="auto auto",
    column_gap="10px",
)
grid = Div(style=style)
grid.children = [
    "index",  Div(children=["#", Index()]),
    "(x, y)", Div(children=["(", ValueRef(field="x"), ", ", ValueRef(field="y")]),
    "radius", ValueRef(field="radius", format="%.2f", formatter="printf"),
]

hover0 = HoverTool(
    tooltips=Template(children=[grid]),
    attachment="left",
)
p.add_tools(hover0)

hover1 = HoverTool(
    tooltips=[
        ("index", "$index"),
        ("(x,y)", "(@x, @y)"),
        ("radius", "@radius{%.2f}"),
    ],
    formatters={
        "@radius": "printf",
    },
    attachment="right",

)
p.add_tools(hover1)

fixes #13599

[mattpap]

When working on this I noticed that the template case only renders tooltip entry for the last selected index, not all of the selected indices as in the default (tuple) case.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.59%. Comparing base (073894a) to head (b5ba116).

Additional details and impacted files

@@             Coverage Diff             @@
##           branch-3.4   #13650   +/-   ##
===========================================
  Coverage       92.59%   92.59%           
===========================================
  Files             325      325           
  Lines           20729    20733    +4     
===========================================
+ Hits            19194    19198    +4     
  Misses           1535     1535