Customize zoom icons for subcoordinate-y plots

[maximlt]

As observed in #6190, it's at the moment not possible to visually distinguish normal wheel-zoom, zoom-in, and zoom-out tools in the toolbar with their subcoordinate-y versions, at least until you hover over them where you can get some more contextualized information. So the suggestion is to create custom icons for these tools when set up for subcoordinate-y plots.

For reference, Bokeh's icons seem to be available in this folder: https://github.com/bokeh/bokeh/tree/branch-3.5/docs/bokeh/source/_images/icons

[droumis]

Relevant: bokeh/bokeh#13826

[droumis]

Should a custom icon go into HoloViews or Bokeh?

I could see how the level and dimensions arguments could potentially be used in Bokeh to customize the tool icon in some way. Alternatively, we could just create a single set of subcoordinate_y icons for HoloViews

[droumis]

@hoxbro : in HoloViews