The movetype's special notes were becoming a new note for the individual unit.
Clean up the documentation on some of movetype's functions, as they had
documentation in both the .hpp and the .cpp.
Fixes#6898. The issue is that non-WML events added through the new events API
always disable undo with no equivalent of WML's `[allow_undo]`. The long-term
fix is to add a way to do that; however until that's available then listeners
for `moveto` need to use the old `on_event` API. The old `on_event` API can't
be deprecated yet, and this is enforced by our unit tests (the build fails if
there are unexpected deprecation warnings during the tests).
Reverts most of 7e234f8833282424b3535b9c334c751748f7222b. Does not revert files
that only listen for non-undoable events such as `die` or `new turn`.
Reverts the deprecation part of #5663's 8cd133263058a5df85f64988e348d2cf54d13a48.
