WIP: Starting work on adding before/after mount directions

This is still a work-in-progress, but this feels like a good point to commit
for safe keeping. This is a non-working WIP.

tests/test_dom_spot.py

@@ -0,0 +1,22 @@
+from textual.widget import Widget
+
+class Content(Widget):
+    pass
+
+class Body(Widget):
+    pass
+
+def test_find_dom_spot():
+    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)
+    assert list(screen.children) == [header,body,footer]
+    assert screen._find_spot(None) == (screen,-1)
+    assert screen._find_spot(1) == (screen, 1)
+    assert screen._find_spot(body) == screen._find_spot(1)
+    assert screen._find_spot("Body") == screen._find_spot(body)
+    assert screen._find_spot("#body") == screen._find_spot(1)