Code style fixes.
Exposed selected methods as a static ones in widget plugin.
Cleaned up and added tests.
Tests: Written basic tests for is* methods.
Tests: Split tests into smaller parts.
Docs: Improved tags.
Added docs for the new methods and sorted them.
Hidden back methods which have no value outside widgets repository.
Removed test which is unnecessary.
More readable elements names.
Split tests into parts.
Removed unnecessary test.
JSCS.
Tests: split Widget.isDomNestedEditable tests.
Changelog entry.
Merge branch 't/12150' into major
First step - generic class pretty much shows the interface, yet without affect to the editor itself.
Tests: Adjusted busindicator tests to use sinon.
Implemented markEditor method.
Reorganized code, updated tests, added integration test, added docs.