fix(widget): fix visual not updated after style changes

[TomJGooding]

Fix a significant regression in Textual v6.0.0 where the widget visual is not updated after style changes.

This effectively reverts the changes in dca9439, which intended to optimize styles caching but clearly had unintended consequences.

Fixes #6235
Fixes #6289
Fixes #6322

[willmcgugan]

Thanks. Going with #6372

[TomJGooding]

@willmcgugan The fix in #6372 doesn't seem to account for the other issues related to the visual style?

For example, notice the label background isn't updated here:

from textual.app import App, ComposeResult
from textual.containers import CenterMiddle
from textual.widgets import Label, Select


class ExampleApp(App):
    def compose(self) -> ComposeResult:
        yield Select.from_values(["red", "green", "blue"])
        with CenterMiddle(id="color-container"):
            yield Label("Try changing the background color")

    def on_select_changed(self, event: Select.Changed):
        self.query_one("#color-container").styles.background = event.value


if __name__ == "__main__":
    app = ExampleApp()
    app.run()