Rename/Restructure robot-testing files

When using ``plone.app.robotramework`` you can re-use keywords from extension packages. If you have your own package relying on ``plone.app.imagecropping`` and you want to check the CroppingEditor in your tests, you could add a Resource in your robot test files as shown below:

```
*** Settings ***
Resource        plone/app/robotframework/selenium.robot
Resource        plone/app/robotframework/keywords.robot
Resource        plone/app/imagecropping/tests/robot/keywords.robot

*** Test cases ***
My sample testcase
    ...
    Open Cropping Editor
    Crop On Image Size  preview  560  20
    Image Must Be Cropped

...
```

This allows you to use keywords from ``plone.app.imagecropping`` in your Test cases.