Make modifying layout easier

Currently it’s not very convenient to change something in a widget layout, especially if you need to change several properties, for example:

w = Button()
w.layout.width = '100px'
w.layout.height = '50px'

It could look much better:

w = Button().with_layout(width='100px', height='50px')

Personally I now use dumb monkey-patching for this:

def _with_layout(self, **kwargs):
    for k, v in kwargs.items():
        setattr(self.layout, k, v)
    return self

iw.DOMWidget.with_layout = _with_layout

and would really like to see something like this in ipywidgets!

[jasongrout]

@gwvr - thanks for bringing this up. This is a separate request from the one on this issue, so I’ve opened a new issue for your comment. Can we continue discussion there? #910.

[jasongrout]

Fixed by #961.