Widget visibility: allow the use of both "Show" and "Hide" for the same widget

---

When I activate visibility, I can choose SHOW or HIDE. Next, every condition are grouped in the same selection.
Imagine this scenario:

```
SHOW if CATEGORY is XXXXX
but
HIDE if TAG is YYYYY
```

I can't use this, because all conditions are grouped in the same selection, SHOW or HIDE.

Suggested here:
- http://wordpress.org/support/topic/widget-visibility-limitations?replies=3

Original trac ticket:
- https://plugins.trac.wordpress.org/ticket/1986

Start by locating the widget visibility implementation and reviewing the linked WordPress support discussion and original Trac ticket. Confirm how visibility conditions are grouped; done means a widget can combine SHOW conditions with HIDE conditions and the resulting behavior is covered by the project's relevant checks.

Written by the indexing model from the issue text.