Treat fonts with the same font descriptor and encoding as aliases

Different fonts can point to the same font descriptor
(see https://github.com/mozilla/pdf.js/issues/4339 for details). With this
commit such fonts are treated as aliases if they have also the same encoding.
The according info is stored on the font descriptor. This change must also
ensure that aliases use always the same font name because translated fonts
can get cleared depending on the CLEANUP_TIMEOUT setting.
Adds make minified command
Makes firefox extension's l10n get() behavior similar to webL10n
Refactor PartialEvaluator: extract font translation

While working on issue #4339 it was confusing that the code to
translate a font is not found in a single place. This commit extracts
the code and puts in a class called FontTranslator