feat: import Rnote notebooks (Issue #47)

The import (menu entry and drag & drop) now also accepts .rnote files
from Rnote 0.5.10 on, including the transformation and document config
layouts that changed in 0.13 and 0.15. Taken over: brush strokes with
their pressure curve, shapes as outlines, texts, bitmap images and SVG
images, which are rasterized with resvg (Rnote stores imported PDF
pages as SVG). Fixed-size and continuous documents keep their pages;
infinite ones become an endless notebook.

The parts shared with the Xournal++ import moved to src/import.rs.
resvg and its dependencies were added to the Flatpak cargo sources.

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

Cargo.toml

@@ -52,6 +52,8 @@ hayro-svg = "0.8"
 flate2 = "1.1"
 quick-xml = "0.41"
 base64 = "0.22"
+# Rnote-Import: SVG-Bilder (z. B. PDF-Seiten) rastern
+resvg = { version = "0.49", default-features = false, features = ["raster-images"] }
 
 # Text-Werkzeug: Schriftmetriken und Rasterung (Schriften aus egui)
 ab_glyph = "0.2"