Unify Accessibility Checker behaviour when no issues are detected

[mlewand]

We should unify behavior of Accessibility Checker when no a11y issues are detected.

Cases

Checking empty document

1. Open any sample with Accessibility Checker. (i.e. plugins/a11ychecker/dev/sample.html)
2. Clear the CKEditor contents, using "New Page" button.
3. Execute Accessibility Checker, eg. by clicking a its toolbar button.

Fixing the last problematic element

1. Open any sample with Accessibility Checker. (i.e. plugins/a11ychecker/dev/sample.html)
2. Set the CKEditor contents using "Source" button to:

<p>This is a sample date: 12-04-12.</p>

1. Execute Accessibility Checker.
2. Navigate to the issue, eg CTRL + E.
3. Apply a Quick Fix.

Expected result:
For the time being alert with an appropriate message will be sufficient.

[mlewand]

Actually we might thing about firing an event in Controller class in case of positive content check.

The event would be like validated or contentValidated.

But the same functionallity can be achieved by using Controller checked event, simply by checking IssueList size (count). This is quite logical and easy to figure out, so I'll not implement it for the time being just to keep it simple.