[Textual] Consider ampersand in queries to refer to the current widget

It may be nice to add a syntax to queries which refers to the current widget. For instance, if I want to get the immediate child of a widget, I could do this: ```python self.query("& > Child") ``` Currently that's rather awkward to implement. We can also extend this to the CSS itself. i.e. ini a `CSS` or `DEFAULT_CSS`. For instance the following would style the widget without the need to specify its type again: ```python DEFAULT_CSS = """ & { border: heavy red} """ ``` Currently, you have to do this: ```python DEFAULT_CSS = """ MyWidget { border: heavy red} """ ```