Support multiple widgets in single table cell

[abalakh]

In some cases we have multiple widgets in single table cell - e.g. 2 buttons in 'Actions' column, but currently Table widget supports only 1 widget per column. We can try to workaround that by moving those widgets into separate view and assigning that view to table cell, but that will create some boilerplate (not to mention no one tried it out so no guarantees it will actually work :) ). Ideally i just want to be able to specify some dict with widgets per table column, e.g:

resources = SatTable(
    locator='//table',
    column_widgets={
        'Version': Text('.//a'),
        'Status': PublishPromoteProgressBar(),
        'Actions': {'clone': Button(id='clone'), 'delete': Button(id='delete')}
    },
)