models: SVG: tree parse to support netsted groups

Instead of looking only at direct childs of a node,
search the complete tree starting at the node.

This is particularly useful when we add fancy attributes,
since they tend to add encompassing groups to the graph.

It is pretty easy to test for the bug and how it can be reproduced.
It's tracked in: hbmartin#45

I re-created a minimal fail case from
hbmartin#45 (comment)

```
digraph {
    Pitas
    Pizza [ tooltip=Delicious ]
    Pitas -> Pizza
}
```

Previous HEAD failed to parse this.

graphviz2drawio/models/SVG.py

@@ -1,14 +1,27 @@
+def svg_tag(tag):
+    return "{http://www.w3.org/2000/svg}" + tag
+
+
 def get_first(g, tag):
-    return g.findall("./{http://www.w3.org/2000/svg}" + tag)[0]
+    target = svg_tag(tag)
+    for i in g.iter():
+        if i.tag == target:
+            return i
+    raise RuntimeError(f"Failed to find tag ({tag}) in {g}, contains {[i for i in get_all(g)]}")
 
 
 def get_title(g):
     return get_first(g, "title").text
 
 
 def is_tag(g, tag):
-    return g.tag == "{http://www.w3.org/2000/svg}" + tag
+    return g.tag == svg_tag(tag)
 
 
 def has(g, tag):
-    return len(g.findall("./{http://www.w3.org/2000/svg}" + tag)) > 0
+    try:
+        get_first(g, tag)
+    except RuntimeError:
+        return False
+    else:
+        return True