Re-organise pyside classes

[dgdekoning]

Issue

While working on #8 I found myself fixing up the imports inside the AB code.

When attempting to run the tests, a circular-import error is shown. I tracked this down to where the ABTab class is being imported from.

Apart from redoing the import statements to do away with relative imports (from ..panels import ABTab should become from activity_browser.layouts.panels.panel import ABTab) I propose to move the ABTab class further up into the structure into a separate base.py file, making it clear that it should be subclassed.