Branches that leave the function are not always clickable

Consider the following small program:

```
global _start

_start:
call main

main:
mov rdi, 42
mov rax, 60
syscall
ret
```

When loading that program into `angr-management`, the `_start` function is displayed like this:


Note that the `main` reference is not clickable (it would have a different color if it was clickable).

The reason for this is that when building the supergraph, edges to blocks outside of the current function are removed. When we later register the out branches of each node, the `_start` node no longer has any outgoing edges (since they got removed in the first step).

It only works correclty if `main` is identified as a `FunctionNode`, but not all branch targets that leave the current function lead to a `FunctionNode`.