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>

README.md

@@ -11,7 +11,7 @@
 
 ---
 
-ink-notes stores handwriting as vectors, can recognize shapes, imports PDFs for annotation and Xournal++ notebooks, inserts images and text boxes, and exports everything again as PDF, PNG or SVG. Saving happens automatically in the background.
+ink-notes stores handwriting as vectors, can recognize shapes, imports PDFs for annotation as well as Xournal++ and Rnote notebooks, inserts images and text boxes, and exports everything again as PDF, PNG or SVG. Saving happens automatically in the background.
 
 It is mainly tested on Zorin OS 18 / Ubuntu 24.04 (Wayland and X11).
 
@@ -49,6 +49,7 @@ The interface is available in English and German; ink-notes follows the system l
 - **Paste with a finger:** Pressing the page with a finger for half a second opens a "Paste" menu – a screenshot, copied text or copied objects land exactly at that spot.
 - **Import** (☰ menu, or drag the file into the window) – **PDF**: Every PDF page becomes a notebook page with the PDF page as background. Portrait pages become A4; landscape pages (e.g. slides) get a landscape page without an empty strip below. When you zoom in, PDF pages are re-rendered more sharply.
   **Xournal++** (`.xopp`, also `.xoj` from Xournal): strokes with pen pressure, highlighters, texts, images, lined/squared/dotted paper and PDF backgrounds are taken over (the PDF is looked for at its old place and next to the file). LaTeX formulas are skipped.
+  **Rnote** (`.rnote`, from Rnote 0.5.10): strokes with pen pressure, shapes, texts and images; PDF pages imported in Rnote become images. Pages stay pages, infinite documents become an endless notebook.
 
 ### Pages and view
 - **Backgrounds:** blank, squared (5 mm), ruled (7.5 mm), dot grid (5 mm). The **color of squares, lines and dots** is selectable (light, medium, strong, blue, green) and also applies to exports.
@@ -172,7 +173,9 @@ src/
 ├── assets.rs       # Images and PDFs in the notebook folder
 ├── pdf_doc.rs      # Reading and rendering PDFs (hayro)
 ├── pdf_merge.rs    # Underlaying imported PDF pages on export
+├── import.rs       # Import as a new notebook (shared parts)
 ├── xopp.rs         # Import of Xournal++/Xournal notebooks
+├── rnote.rs        # Import of Rnote notebooks
 ├── document.rs     # Data model: notebook, pages, strokes, page sizes
 ├── history.rs      # Undo/redo
 ├── storage.rs      # Loading/saving (atomic), notebooks, trash