[RELEASE] Clean-Up and Slim Down

Clean-up release. Notice the following breaking changes:

• RenderingContextFixture has been removed (use regular mocking for your tests)
• FileStreamWrapper has been removed (use vfs stream instead)
• Accessible mocks no longer offer _setRef, _callRef, _setStatic, _getStatic
• The BaseTestCase no longer offers inject and callInaccessibleMethod (use DI directly, use different mocking solutions)

Changelog since 5.0.16:

• [!!!][TASK] Remove FileStreamWrapper
• [TASK] Raise minimum phpunit version
• [TASK] Clean up specific ViewHelper functionality
• [BUGFIX] Streaming scenario creation
• [!!!][TASK] Make AccessibleObject only work with call/get/set
• [TASK] Use trait for AccessibleProxy
• [!!!][TASK] Drop support for "callInaccessibleMethod" and "inject"