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/i18n.rs

@@ -167,6 +167,8 @@ mod tests {
         ("text.rs", include_str!("text.rs")),
         ("ui.rs", include_str!("ui.rs")),
         ("updater.rs", include_str!("updater.rs")),
+        ("import.rs", include_str!("import.rs")),
+        ("rnote.rs", include_str!("rnote.rs")),
         ("xopp.rs", include_str!("xopp.rs")),
     ];