Register Windows file associations properly

Fix two defects:
1. Use proper ProgID in the form Poedit.*
2. Don't uninstall extension registry key

See http://msdn.microsoft.com/en-us/library/windows/desktop/hh127451(v=vs.85).aspx
More compact Finder/Explorer filetype kind strings

Don't say "PO Translation File", say just "PO Translation" - that "file"
is redundant and native apps tend not to use it.
Windows: Localize Explorer file kind strings
Update lproj handling in Crowdin script

Fix after changed location.
Fix copyright string in InnoSetup script
Fix wrong support email in InnoSetup script
Use InnoSetup translations

Only enable the officially maintained InnoSetup translations, don't
integrate with Crowdin or use custom translations for other languages -
it's not worth the hassle for just an installer.
Fix crash when updating from sources w/ sidebar on

Fixes #121.