fix: Plotly express widgets don't work in deephaven.ui

[jnumainville]

fixes #450

Plotly express' Figure object is unhashable, so it couldn't be a dictionary key.

Tested with the following snippet:

df = px.data.tips()
fig_violin = px.violin(df, y="tip", x="smoker", color="sex", box=True, points="all", hover_data=df.columns)
ui_fig_violin = ui.panel(fig_violin)

[mofojed]

I was worried when I was reading this: https://docs.python.org/3/library/functions.html#id

Two objects with non-overlapping lifetimes may have the same id() value.
But I guess this can not be a problem since an object will be held in the _old_objects set for it's lifetime.