This patch doesn't add any new functionality and keeps the introductory
tutorial and economy walkthrough as they are. Instead, it rearranges some
code to enable easily adding different types of tutorial steps in the
future. The idea is for each type to be displayed on a different panel
and for the TutorialManager to switch back and forth between them, and
to handle and translate the received messages from the simulation and
pass them along to the active panel. Currently, there are only
"instruction"-type steps, which are handled by the InstructionPanel in
the GUI. But new ones can be added in the future, like information boxes
or bigger objectives.
While it's currently the case everywhere, tutorial "steps" don't
necessarily have to be goals, they could be stuff like tips or info too
in general, if different types of "steps" are added in the future.
