Add resource strings for file associations

These weren't merged from 2.1.9 with the rest of the file association
code, which results in the installer trying to reference nonexistent
resources and things breaking.

Updates #1699.
Set ApplicationCompany for the Aegisub executable

Despite being completely unmentioned in the documentation this appears
to be required to show up in the default programs UI on Windows 8.1.

Closes #1699.