[i18n, feat] Add basic context (msgctxt) support (#5234)

References https://github.com/koreader/koreader/issues/5232

Given an entry in the PO file like the following:

```
msgctxt "systemstat"
msgid "    Total"
msgstr "Totaal"
```

It can be addressed using:

```lua
local _ = require("gettext")
local C_ = _.pgettext
C_("systemstat", "    Total")
```

This allows to distinguish between separate instances of the same string, for example "Pages" meaning "Number of pages" and "Pages" meaning "Display of pages".

Extraction of this code pattern is not yet implemented by nightswatcher. xgettext didn't yet support Lua back in 2013 when all this was first added to the program, but now it does. Therefore it might make the most sense to replace the current Python extraction script with xgettext itself.

By default it only understands gettext.pgettext(), but that can be addressed by passing some extra command line arguments, for example:

```
xgettext -l lua -c --keyword=C_:1c,2 *.lua
```

frontend/gettext.lua

@@ -12,8 +12,8 @@ local GetText_mt = {
     __index = {}
 }
 
-function GetText_mt.__call(gettext, string)
-    return gettext.translation[string] or string
+function GetText_mt.__call(gettext, msgstr)
+    return gettext.translation[msgstr] or msgstr
 end
 
 local function c_escape(what)
@@ -36,6 +36,7 @@ end
 -- we only implement a sane subset for now
 
 function GetText_mt.__index.changeLang(new_lang)
+    GetText.context = {}
     GetText.translation = {}
     GetText.current_lang = "C"
 
@@ -61,7 +62,15 @@ function GetText_mt.__index.changeLang(new_lang)
         local line = po:read("*l")
         if line == nil or line == "" then
             if data.msgid and data.msgstr and data.msgstr ~= "" then
-                GetText.translation[data.msgid] = string.gsub(data.msgstr, "\\(.)", c_escape)
+                local unescaped_string = string.gsub(data.msgstr, "\\(.)", c_escape)
+                if data.msgctxt and data.msgctxt ~= "" then
+                    if not GetText.context[data.msgctxt] then
+                        GetText.context[data.msgctxt] = {}
+                    end
+                    GetText.context[data.msgctxt][data.msgid] = unescaped_string
+                else
+                    GetText.translation[data.msgid] = unescaped_string
+                end
             end
             -- stop at EOF:
             if line == nil then break end
@@ -91,6 +100,10 @@ function GetText_mt.__index.changeLang(new_lang)
     GetText.current_lang = new_lang
 end
 
+function GetText_mt.__index.pgettext(msgctxt, msgstr)
+    return GetText.context[msgctxt] and GetText.context[msgctxt][msgstr] or msgstr
+end
+
 setmetatable(GetText, GetText_mt)
 
 if os.getenv("LANGUAGE") then