Minimize the first level imports (#105)

All classes that are not essential for creating exercises have been
removed from the first level import.

Author: agoscinski

src/scwidgets/__init__.py

@@ -8,27 +8,14 @@
 
 __all__ = [  # noqa: F405
     # cue
-    "CueWidget",
-    "CheckCueBox",
-    "CueBox",
-    "SaveCueBox",
-    "UpdateCueBox",
-    "ResetCueButton",
-    "SaveResetCueButton",
-    "CheckResetCueButton",
-    "UpdateResetCueButton",
     "CueOutput",
     "CueObject",
     "CueFigure",
     # code
     "CodeInput",
     "ParameterPanel",
     # check
-    "Check",
-    "CheckResult",
-    "AssertResult",
     "CheckRegistry",
-    "CheckableWidget",
     "assert_equal",
     "assert_shape",
     "assert_numpy_allclose",
@@ -38,6 +25,5 @@
     # exercise
     "CodeExercise",
     "TextExercise",
-    "ExerciseWidget",
     "ExerciseRegistry",
 ]