Robustify activation of tools/proxies in merged toolbars

[mattpap]

This PR:

• fixes handling of active_ properties when merging toolbars
• allows to use ToolProxy in active_ properties
• allows to activate a proxy with a single tool that belongs to that proxy

Screencast.from.09.05.2024.12.55.00.webm

Code

from bokeh.plotting import figure, show
from bokeh.layouts import gridplot

tools = "pan,wheel_zoom,box_zoom,reset,tap,crosshair,save,box_select"
plots = [figure(active_drag="box_select", tools=tools, width=200, height=200) for _ in range(2)]
for p in plots:
    p.scatter([1, 2, 3], [1, 2, 3], size=10)
gp = gridplot(children=plots, ncols=3, merge_tools=True)
show(gp)

fixes #8766

[philippjfr]

Looks good to me!