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These translations being out of date won't pose any technical problems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

.gitignore

@@ -7,7 +7,7 @@ doc-build/
 
 # Generated source files
 src/common/scm_rev.cpp
-.travis.descriptor.json
+dist/english_plurals/generated_en.ts
 
 # Project/editor files
 *.swp