Translate english plurals

Turns out that for Qt to properly handle plurals in English a
translation needs to be provided, otherwise the user is left with
messages such as "Building: 2 shader(s)"

Plurals for other all other languages are handled on transifex.
I wrote the README.md to just refer to it as a translation
collaboration site just in case we ever switch.

These translations being out of date won't pose any technical problems
so I believe it is fine to handle them manually on a "best effort"
basis.

The files are generated into the source directory so that the
relative filenames are correct. The generated file is added to
.gitignore

.reuse/dep5

@@ -2,7 +2,8 @@ Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
 Comment: It is best to use this file to record copyright information about
          generated, binary and third party files
 
-Files: dist/icons/controller/*.png
+Files: dist/english_plurals/*
+       dist/icons/controller/*.png
        dist/icons/overlay/*.png
        dist/languages/*
        dist/qt_themes/*/icons/index.theme