Lots of reworking of the mount-before/after work

Lots of things going on here, mainly narrowing in on the final form.

tests/test_widget_mount_point.py

@@ -0,0 +1,40 @@
+import pytest
+
+from textual.widget import Widget
+
+
+class Content(Widget):
+    pass
+
+
+class Body(Widget):
+    pass
+
+
+def test_find_dom_spot():
+
+    # Build up a "fake" DOM for an application.
+    screen = Widget(name="Screen")
+    header = Widget(name="Header", id="header")
+    body = Body(id="body")
+    content = [Content(id=f"item{n}") for n in range(1000)]
+    body._add_children(*content)
+    footer = Widget(name="Footer", id="footer")
+    screen._add_children(header, body, footer)
+
+    # Just as a quick double-check, make sure the main components are in
+    # their intended place.
+    assert list(screen.children) == [header, body, footer]
+
+    # Now check that we find what we're looking for in the places we expect
+    # to find them.
+    assert screen._find_mount_point(1) == (screen, 1)
+    assert screen._find_mount_point(body) == screen._find_mount_point(1)
+    assert screen._find_mount_point("Body") == screen._find_mount_point(body)
+    assert screen._find_mount_point("#body") == screen._find_mount_point(1)
+
+    # Finally, let's be sure that we get an error if, for some odd reason,
+    # we go looking for a widget that isn't actually part of the DOM we're
+    # looking in.
+    with pytest.raises(Widget.MountError):
+        _ = screen._find_mount_point(Widget())