Add support for modifiers and allow to auto-activate WheelZoomTool

[mattpap]

Originally I started working on scroll support in BoxAnnotation per issue #13646, but I realized that I need to resolve the more generic issue of conditional handling of scroll events first, so that bokehjs doesn't interfere by with page scrolling, while allowing some way of easy access to scrolling functionality (like wheel zoom or wheel pan tools provide).

Thus this PR:

• redesigns how tool event handlers are invoked and allows to indicated from such handlers if an event was handled or not (this has the added benefit of not using expensive signaling infrastructure to dispatch UI events to tool handlers)
• adds support for modifiers to WheelZoomTool and WheelPanTool
• adds support to Toolbar for auto-activation of such tools when modifiers are set
• adds support for a visual indicator that a modifier is required to perform tool's function (e.g. an alert or a plot overlay)

This example shows WheelZoomTool configured with modifiers = dict(ctrl=True), first being scrolled without the modifier and then with:

Screencast.from.09.04.2024.02.22.41.webm

fixes #10439

[jbednar]

Cool! Can you describe the details of how it "redesigns how tool event handlers are invoked"?

[mattpap]

Cool! Can you describe the details of how it "redesigns how tool event handlers are invoked"?

The previous approach using signals didn't allow for returning a value from handlers (like WheelZoomTool._scroll), so this PR changes that, replacing signals with direct function calls.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.64%. Comparing base (e61219b) to head (d5ccbf9).
Report is 1 commits behind head on branch-3.5.

Additional details and impacted files

@@              Coverage Diff               @@
##           branch-3.5   #13815      +/-   ##
==============================================
+ Coverage       91.57%   92.64%   +1.06%     
==============================================
  Files             326      326              
  Lines           20737    20751      +14     
==============================================
+ Hits            18990    19224     +234     
+ Misses           1747     1527     -220