Replace the standard Lua include path entirely on Windows

The default Lua include path is encoded with the local charset rather
than UTF-8. This causes problems when the working directory includes
non-latin-1 characters which are valid in the local charset, as Aegisub
expects the string to be UTF-8. As the default path isn't very useful on
Windows (on other platforms it could potentially include things like
luarocks-installed modules), just clear it.

Closes #1676.
Fix potential use-after-frees on redo

Don't delete the old copy of the file until after the new version is
committed to avoid a use-after-free when code tries to do things with
the active line in a COMMIT_NEW handler, as just trying to avoid doing
that hasn't really worked out.
Update pt_PT.po due to 3.1.0 changes
Merge pull request #7 from Leinad4Mind/patch-2

Update pt_PT.po due to 3.1.0 changes