fix: support nested output widgets in voila executor

[maartenbreddels]

Fixes #355

The issue boiled down to having nest output widgets like:

import ipywidgets as widgets
output1 = widgets.Output()
output2 = widgets.Output()
with output1:
    print('in 1')
    with output2:
        print('in 2')
    print('also in 1')

Voila would not keep a stack of output hooks, but only the last one. This caused it to fail after the output1 context was left.

The solution is to keep a stack of output hooks, tests are included.

[jtpio]

Thanks @maartenbreddels.

Tested locally with the example above and the nested outputs are handled properly.