Add custom exceptions and add error handling to the engine's module
loading logic. I.e. when a non-existent module is loaded, an exception
is thrown now.
see #23
Add basic drawing of widgets. Each module instance returns a list of
widgets using the widgets() method which is then forwarded to the draw()
method of the configured output.
see #23
Allow the user to provide aliases when loading a module multiple times
so that the modules can be differentiated (e.g. for passing parameters
to a module).
see #23
This is going to be a bit more comprehensive than anticipated. In order
to cleanly refactor the core and the engine, basically start from
scratch with the implementation.
Goals:
* Test coverage
* Maintain backwards compatibility with module interface as much as
possible (but still make modules easier to code)
* Simplicity
see #23
