Refs #4189, bug fixes for dashboard & changes to make dashboard UI tests possible. Includes:

- fix for bug where top controls not displayed correctly when dashboard is displayed in a widget iframe.
- do not display dashboard manager after dashboard action selected.
- truncate tables when running UI tests w/ --persist-fixture-data so existing database data won't be used if the fixture setup didn't complete before.
- do not create super user more than once in Fixture.
- don't include Test_Piwik_Fixture_CustomAlerts in omni fixture.
- make sure UITestFixture executes the correct code every time regardless of whether --persist-fixture-data isn't used.
- add mouseup/mousedown events to screenshot testing page rendere.
- add ability to call controller methods in screenshot tests and fix API calling mechanism.