Move "empty" definition to test.helpers.
Tests: greatly expand the UI tests.
Tests: replace NULL with {ui,m}Interface.

Replace all occurrences of ffi.NULL with {ui,m}Interface, since that is what
will be passed by the DSP C code.
PythonUI: fix a typo in addToggleButton().

Fix a typo in addToggleButton() which led to "c_label" being passed to the
Python function instead of "label".
PythonUI: simplify code a bit.