page view: add baselines if available

For handwritten text, as for any segmentation produced via baseline detection, it is usually helpful to see the detected baselines directly. We should make that available next to

browse-ocrd/ocrd_browser/view/page.py

Line 152 in cd34055

    
           Feature.LINES: FeatureDescription('𝌆', 'lines', '/page:PcGts/page:Page/*//page:TextLine/page:Coords'),

by something like

    Feature.BASELINES: FeatureDescription('–', 'baselines', '/page:PcGts/page:Page/*//page:TextLine/page:Baseline'),

These coordinates are linestrings instead of linear rings / polygons, and we may need to use a stronger line width when drawing to make it visible. But other than that, it should be fairly simple.

Wonderful, thank you!