feat: import Xournal++ and Xournal notebooks (Issue #47)

The import menu entry (formerly "Import PDF") and drag & drop now also
accept .xopp and .xoj files. Each Xournal page becomes a page of a new
notebook: strokes with pen pressure, highlighters, texts, embedded
images, lined/squared/dotted paper, image backgrounds and PDF
backgrounds (looked up at the stored path and next to the file). All
layers are flattened in order; LaTeX formulas are skipped and counted
in the status message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/i18n.rs

@@ -167,6 +167,7 @@ mod tests {
         ("text.rs", include_str!("text.rs")),
         ("ui.rs", include_str!("ui.rs")),
         ("updater.rs", include_str!("updater.rs")),
+        ("xopp.rs", include_str!("xopp.rs")),
     ];
 
     /// Alle Texte in `t!("…")` und `tn!("…")` eines Quelltexts, auch über mehrere Zeilen