Use single interface in the screenshot tests

[Nikitae57]

DocLocScreenshotTestCase does not reuse the Screenshot interface. Instead it uses it's private implementation (this seems to be a very unexpected behavior, this approach may cause confusion, since redefining the basic interface does not affect the operation of dock&lock tests in any way).

This issue is a proposal to solve this problem by unifying the Screenshot interface by adding a new method there and then migrating DocLocScreenshotTestCase to use the Screenshot interface.