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>

src/lib.rs

@@ -13,12 +13,14 @@ pub mod geometry;
 pub mod history;
 pub mod i18n;
 pub mod image_layer;
+pub mod import;
 pub mod library;
 pub mod osk;
 pub mod page_export;
 pub mod pdf_doc;
 pub mod pdf_merge;
 pub mod platform;
+pub mod rnote;
 pub mod ruler;
 pub mod selection;
 pub mod settings;