Implement a GUI to view labelled transition systems

[mlaveaux]

This is currently being done using Slint, but there are still a few features lacking compared to ltsgraph.

• [x] The graph is not drawn on the whole area available (it is truncated)
• [x] Pause simulation when the graph is stable
• [x] Add toggle to halt simulation.
• [x] Drawing with tiny-skia is fairly efficient, but perhaps switching to femtovg or even wgpu could improve it further.
• [x] Zooming does not really work consistently, it should zoom from the center of the screen and the labels should stay in place
• [x] Add arrows to the edges (they should be directed edges).
• [x] Show self loops as a circle, and perhaps also allow other edges to bend if that is useful?
• [x] Pause simulation and drawing when no LTS is loaded.
• [x] The file dialog (rfd) does not show all the files for some directories.
• [x] Show the logging information in a (separate) window, especially on Windows where the default terminal output will not be shown.
• [x] Show the initial state with a separate color.
• [x] Allow zooming out furher

[mlaveaux]

For now just keep tiny-skia although technically abandoned. It seems like femtovg might be an alternative with no performance benefit. The only alternative is using the real Skia library with Rust bindings or switch to GPU rendering with wgpu.