web: Add visual progress indicator to capture screen
web: Add visual progress indicator to capture screen

Only displayed when the 'extent' field in the workflow metadata is a
numerical value (= number of pages expected).

miloh/spreads,gareth8118/spreads,adongy/spreads,adongy/spreads,miloh/spreads,miloh/spreads,DIYBookScanner/spreads,gareth8118/spreads,nafraf/spreads,DIYBookScanner/spreads,nafraf/spreads,DIYBookScanner/spreads,adongy/spreads,gareth8118/spreads,nafraf/spreads