Update mx_AppLoading to improve maintainability

[luixxiul]

For element-hq/element-web#25269

This PR suggests to update mx_AppLoading of _AppsDrawer.pcss to:

• Improve maintainability by nesting the rules, to prevent the rule from becoming an orphan, which can only be detected by manual labor
• Clarify that AppLoading belongs to AppTile.tsx to extract its style rules from AppDrawer.pcss
• Conform class name to the naming policy

type: task

Signed-off-by: Suguru Hirahara [email protected]

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.