feat(core,gui): gettext translations and language preference

The app, the tray and the search prompt route every user-visible string
through GNU gettext in the `pdfs` text domain, with named placeholders and
plural forms. po/ holds the template, update and build scripts, and catalogs
for de, en_GB, es, fr, it, nl, pl and pt_BR. Preferences gains a Language row
that overrides the system language from the next start (config `language`).

Packages compile the catalogs into /usr/share/locale and the desktop files
carry translated descriptions and keywords. docs/TRANSLATING.md describes
the workflow.

crates/pdfs-gui/Cargo.toml

@@ -35,3 +35,5 @@ adw = { package = "libadwaita", version = "0.9", features = ["v1_5"] }
 async-channel = "2"
 exif = { version = "0.6", package = "kamadak-exif" }
 webkit6 = "0.6"
+# The system libintl (glibc) rather than a bundled gettext build.
+gettext-rs = { version = "0.7", features = ["gettext-system"] }