page view: add baselines if available

[bertsky]

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.

[bertsky]

Wonderful, thank you!