feat: add inkml parser and writer

[Doublonmousse]

Related to #744 (import of selected onenote strokes, but exclusive to that. No text nor images are read).

As I'm on windows, I'm using #1282 for now.
The parser/writer crate is on a separate repository here https://github.com/Doublonmousse/writer_reader_inkml

Export and import are supported (from the clipboard).

• I think it would make sense to move some of the import code inside the engine (conversion from inkml to stroke ?)
• The export part is only for brush strokes (I could extend it to shapes with lines being the easiest to do, and other shapes would have to be discretized)
• I'm using the windows equivalent of the mime type for the inkml clipboard export part
• The crate still needs some polishing (better error message and removing some potentially panic inducing unwrap with error propagation instead).

I've encountered a weird behavior with clipboards in gtk and windows (that ofc I can't reproduce today ...). Copying ink content then an image from a source application wouldn't work as somehow gtk would think the inkml format was still present before failing with a gio quark error.