Fix nested modal support

[mvorisek]

fixes #1732
fixes #1699
and highly improves the Behat testing quality

Steps to reproduce:

1. open debug example demos/_unit-test/crud_gh1732.php
2. click "flag" icon on the 1st row
3. click "edit" icon in the opened modal and notice Mustard name
4. click outside the modal to close it (the Product modal)
5. click outside the modal to close it (the Category modal)
6. repeat 2. step with 2nd row
7. repeat 3. step, notice, there is still Mustard name instead of Cola