Retired: Add a switch to isolate a widget class from CSS type inheritance

[davep]

This PR seeks to satisfy #2749.

It introduces an isolate_css_type switch (name still to be finally decided upon, although I think this carries the correct intent), whose default is False, which when set to True will isolate the new widget type within CSS. While it will inherit all of the mechanics of its ancestors, it will inherit none of their styling and won't be queryable via an ancestor type.

With this PR, this code:

from textual.app        import App, ComposeResult, RenderResult
from textual.containers import Horizontal, Vertical
from textual.widgets    import Static, TextLog

class CustomParent( Static ):

    def __init__( self ) -> None:
        super().__init__()
        self.border_title = self.__class__.__name__

    def render(self) -> RenderResult:
        return str( self._css_type_names )

class CustomChild( CustomParent ):
    pass

class CustomGrandChild( CustomChild, isolate_css_type=True ):
    pass

class CSSTypeExplorerApp( App[ None ] ):

    CSS = """
    Static {
        border: panel green;
    }

    CustomGrandChild {
        border: panel red;
    }
    """

    def compose( self ) -> ComposeResult:
        with Horizontal():
            with Vertical():
                yield CustomParent()
                yield CustomChild()
                yield CustomGrandChild()
            yield TextLog()

    def on_mount( self ) -> None:
        for static in self.query( Static ):
            self.query_one( TextLog ).write( f"{static!r}" )

if __name__ == "__main__":
    CSSTypeExplorerApp().run()

produces this:

(Note Tooltip appearing here, which relates to a motivation behind this quite nicely - #2723)

[davep]

See this comment on the originating PR. I've since noticed that Textual seems to already have a facility to do this anyway. Going to double-check I'm not missing anything obvious here but I think this PR can be closed as it's aiming to duplicate existing functionality that we forgot we had.

[davep]

Confirmed: inherit_css is this anyway and it seems we just forgot we had this. Closing this PR as it's not necessary; and Tooltip can be fixed with this.