Refactor persist_context to correct subnamespace crash and avoid holding stale config references in general.
Add persistent variable subnamespace usage into scenario-test, for testing persistent variable subnamespace behavior.
Compatibility patch for refactored persist_context in wesnoth 1.10
Update changelog to mention fix for bug #20385
Add a missing include that only MSVC10 cares about
Fix a typo in DiD.

The issue was discovered by vultraz.
updated Russian translation
updated Chinese (Traditional) translation
updated Polish translation
added typo fix from mordante to pofix.py and applied the script
pot-update and regenerated doc files, no new/fuzzy strings
updated Polish translation credits
Change some implicit conversions to return const references to objects...

...instead of temporary objects.

Fixes bug #20360.
Merge branch '1.10' of github.com:wesnoth/wesnoth-old into 1.10