v6.4: Save parser/processor fixes, and displaying warnings in UI

What's Changed

• Fix save parsing fails with values starting with nan by @MichaelMakesGames in #166
• Fix memory leak in save processing related to shared_description cache by @MichaelMakesGames in #166
• Handle missing leaders by falling back to "an unknown leader" in event ledger by @MichaelMakesGames in #166
• Fix leader deleted from DB by @MichaelMakesGames in #169
• Add notifications to the main screen for no games and invalid settings by @MichaelMakesGames in #170
• Fix escaped backslash at end of quoted string erroneously escapes the final quote by @MichaelMakesGames in #171

Full Changelog: v6.3...v6.4