[i18n] Switch make pot to xgettext (#5237)

This introduces context to our strings to differentiate them when necessary.

The syntax chosen is `C_()`, following [glib](https://developer.gnome.org/glib/2.28/glib-I18N.html#C-:CAPS).

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

C_("Menu|StyleTweaks|", "Pages")
```

Closes #5232.

frontend/ui/data/css_tweaks.lua

@@ -10,10 +10,11 @@ They may have the following optional attributes:
 ]]
 
 local _ = require("gettext")
+local C_ = _.pgettext
 
 local CssTweaks = {
     {
-        title = _("Pages"),
+        title = C_("Style tweaks category", "Pages"),
         {
             id = "margin_body_0";
             title = _("Ignore publisher page margins"),