Replace status window by something more stylish

[amolenaar]

Is your feature request related to a problem?

When loading a (large) model, Gaphor shows a status window that shows the progress of loading such a model. The status window looks a bit out of place with the new styling.

Describe the solution you'd like

I think there are a couple:

1. show the progress bar in the center part of the page, where the diagrams are placed. The rest of the page should be made insensitive then.
2. Show a dialog without buttons, or (maybe better) with a cancel button, so users have the option to cancel model loading.
3. An overlay on the main window that shows a progress bar.

Describe alternatives you've considered

Additional context

We may consider that it's important to support intermediate (quick) save operations

Current behavior:

Screencast.from.2024-04-24.15-58-14.mp4