For some reason "project-save" isn't emitted when closing project - see
write_config(FALSE) in project_close(). This means that in this case
plugins cannot save their configuration into the config file. This doesn't
even correspond to the documentation of the signal

"Sent when a project is saved (happens when the project is created, the
 properties dialog is closed or Geany is exited)"

as the signal isn't emitted when exiting Geany because at this point Geany
closes the project.

The comment seems to indicate that the reason is that "project-save"
shouldn't be emitted when "project-close" is emitted but I don't see any
reason why.

Bump API so plugins can rely on the changed behavior.