allow to specify translation context

Is your feature request related to a problem? Please describe. Sometimes you need to have separate translations for the same source strings.

Describe the solution you'd like This is accomplished by translation contexts, eg

#: src/gui/widgets/preferences.c:858
msgctxt "guilabel"
msgid "Plugins"
msgstr "プラグイン"

Here, the string "Plugins" is for the translation context "guilabel". There might be another string "Plugins" for another translation context, where the translation will be different.

There should be a way to optionally specify translation contexts, either, per-string, or per-document, or for the whole project that sphinx is translating.

Describe alternatives you've considered N/A

Additional context See Contexts in gettext manual: https://www.gnu.org/software/gettext/manual/html_node/Contexts.html