Merge show_wml_errors() and show_wml_message().
Collect some parts of pump.cpp into a new class (context).

This has no immediate benefit other than organizing things.
Maybe it will help someone do something someday.
Refactor static_wml_actions as class wml_action.

It's a bit more C++-like, but more importantly puts related static
variables in the same file (so this should cure the static
initialization fiasco known as bug #21020).