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.lock

@@ -1397,10 +1397,12 @@ version = "0.35.0"
 dependencies = [
  "ab_glyph",
  "arboard",
+ "base64",
  "bytemuck",
  "egui",
  "egui-wgpu",
  "egui-winit",
+ "flate2",
  "fs2",
  "glam",
  "hayro",
@@ -1411,6 +1413,7 @@ dependencies = [
  "octotablet",
  "pollster",
  "printpdf",
+ "quick-xml",
  "semver",
  "serde",
  "serde_json",