Fix 'trusted-create-element' — do not add element on every DOM changes

[AdamWr]

Currently element is added every time when there is change in DOM.

Steps to reproduce:

1. Add these rules:

example.org#%#//scriptlet('trusted-create-element', 'body', 'a')
example.org#%#//scriptlet('trusted-create-element', 'body', 'div')

2. Go to - https://example.org/

Website is unresponsive because elements are added in the loop.

---

Consider also a case when one element is added inside another:

example.org#%#//scriptlet('trusted-create-element', 'body', 'foo')
example.org#%#//scriptlet('trusted-create-element', 'body > foo', 'bar')