Clear package search queries when closing to the system tray

[Gabriel Dufresne (GabrielDuf)]

This pull request introduces a new feature that allows all search queries across different pages to be cleared at once, and ensures that search queries are reset when the main window is closed (but not fully exited). The main changes involve adding clear methods to the relevant view models and integrating this functionality into the window closing logic.

New search query clearing feature:

• Added a ClearAllSearchQueries method to MainWindowViewModel that clears search queries on DiscoverPage, UpdatesPage, and InstalledPage, and resets the global search text.
• Added a ClearSearchQuery method to PackagesPageViewModel that resets the search query state and triggers filtering if necessary.

Integration with window closing behavior:

• Modified OnClosing in MainWindow.axaml.cs to call ViewModel.ClearAllSearchQueries() when the window is hidden (e.g., minimized to tray), ensuring that search states are reset.