Improve loading state while seeking table

[mofojed]

When scrolling with a large table from a slow backend (or has slow to compute rows), if the data takes a long time to come in, the grid just appears blank and there is no loading indicator. We should have some sort of subtle loading indicator to show there is still a request in progress, "Waiting for viewport...".

Suggested fix:
Display a status message (the loadingtext at bottom of table) with a delay of ~0.5s before showing. We don’t want the loader to show in most scenarios, because it’s normally snappy. Show the text “Waiting for viewport…” with the back and forth loader.

Example query (slow when scrolling because of sleep):

from deephaven import empty_table
import time

def f(a):
    time.sleep(0.1)
    return a

t = empty_table(1000).update("X = i").update_view("Y= f(X)")