added textual.getters

[willmcgugan]

Good for DRY

• Adds getters.query_one
• Adds getters.child_by_id

These create nicely typed properties which return a Widget.

class MyWidget(Widget):
    @property
    def my_input(self) -> Input:
        return self.query_one("#my_input", Input)

Can be replaced with:

class MyWidget(Widget):
    my_input = getters.query_one("#my_input", Input)

Works nicely, just don't look behind the curtain.

[python-and-novella]

I find a mistake in error info.

[python-and-novella]

I think the id should be self.child_id.

[python-and-novella]

I think the id should be self.child_id.

[python-and-novella]

query_one's __get__ is using obj.query_one,
I think it is maybe better for child_by_id's __get__ to use obj.get_child_by_id.