Keep merging elements, but remember the boundaries in the debug info, separated by a slash.
Also fixed tests that rely on accessible-label being set only once. For example
```
Button { text: "foo"; }
```
will certainly have "foo" as accessible-label on `Button`, but its internal `Text` element has
an implicit "accessible-label" set to the same
value.
So don't rely on that for now but search by id instead.
