fix: spawn async event loop on gtk to prevent delayed messages (fix #132)

[aschey]

Resolves #132

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Documentation
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
This change spawns an async event loop on GTK's main thread to process incoming messages from the backend instead of using the same loop that calls gtk::main_iteration(). This allows messages to be processed quickly even when the user isn't interacting with the app because messages won't be impeded while waiting for gtk::main_iteration() to complete.

[wusyong]

Oh I forgot to mention I tested on tauri example and it seems working fine.

[aschey]

Awesome, thank you!