Port GtkSpinner to AdwSpinner

AdwSpinner is a modern replacement for GtkSpinner.

See also: Teams/Design/os-mockups#202 (closed)

• GsLoadingPage: !2086 (merged)
• GsUpdatesPage: !2086 (merged)
• GsAppRow
• GsInstalledPage
• GsScreenshotImage
• GsSearchPage
• GsReposDialog
• GsUpdateDialog
• GsExtrasPage
• GsAppDetailsPage