Fixed error on add to container procedure.
Added run test in button test. Improved docstrings in grid test.
Added is_loaded flag to Viewer as a property.
Made major change in loading API.

- Widget now has a load_graphics specific for graphics.
- Containers now have a load_children specific for content.
- Viewer's load is now used to load each load.
- Viewer's load changes is_loaded property, and enforces it.
Simplified load and unload of graphics. Updated tests.

Load API works correctly: case tested and example tested.
Fixed two examples with new loading API.
Made every container a subclass of Container.

Besides, fixed an error on Dialog and removed load_graphics from Viewer.
Merged Widget with Viewer. Now only Widget exists inside core.

Rectangle moved to core.
Renamed Widget to Viewer.