Make pn.depends and pn.interact accept widgets

[jsignell]

Closes #638

These all work now:

ticker = pn.widgets.Select(name='Ticker', options=tickers)
window = pn.widgets.IntSlider(name='Window Size', value=6, start=1, end=21)

@pn.depends(ticker, window)
def get_plot(ticker, window_size):
    df = get_df(ticker, window_size)
    return df.hvplot.line('date', 'close', grid=True)

pn.Row(
    pn.Column(title, ticker, window),
    get_plot
)

pn.interact(get_plot, ticker=ticker, window_size=window)

pn.interact(get_plot, ticker=ticker.param.value, window_size=window.param.value)

[jsignell]

I haven't fixed all the examples or written tests, just wanted to check with @philippjfr that this seems sane and helpful.

[philippjfr]

It looks like you didn't push the depends.py module.

[jsignell]

I don't really think there is any value in interact accepting widget.param.value in addition to widget, so I got rid of all the changes in interact