This preparation commit only moves code around and renames it, the
history is still a list of plug-ins only:
- move app/core/gimp-filter-history.c
to app/plug-in/gimppluginmanager-history.c and clean it up
- move the actions that create the submenus under "Filters"
from the "plug-in" to the "filters" action group
- move the code that creates and updates the history actions
to the "filters" action group
- add menu setup code for the "filters" menu
- move the "history-changed" signal from GimpPlugInManager to Gimp