Add a stop button in the status bar next to the pending requests counter. When active requests are pending, the button is enabled and allows the user to cancel all ongoing network operations.

• Add WebService.stop_and_notify() that aborts active requests while calling handlers with the error so albums transition to ERROR state
• Add Album.abort_loading() to cleanly abort and finalize loading
• Add stop button (SP_BrowserStop icon) to InfoStatus widget
• Rename InfoStatus widget names from label1-5/val1-5 to meaningful names (eta_icon, files_value, pending_requests_icon, etc.)
• Connect stop_requested signal from InfoStatus to MainWindow