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>

Cargo.toml

@@ -48,6 +48,11 @@ fs2 = "0.4"
 hayro = "0.8"
 hayro-svg = "0.8"
 
+# Import von Xournal++-Notizbüchern (gzip-komprimiertes XML mit base64-Bildern)
+flate2 = "1.1"
+quick-xml = "0.41"
+base64 = "0.22"
+
 # Text-Werkzeug: Schriftmetriken und Rasterung (Schriften aus egui)
 ab_glyph = "0.2"