Allow configuring OWC layers *not* in getCapabilities

**Is your feature request related to a problem? Please describe.**
When configuring a WMS/WFS getMap/getFeature probe, GHC shows a dropdown with the available layers. If you want to monitor 'hidden' layers (i.e. not in the getCapabilities document, like the 'non-advertised' layers of GeoServer), you cannot select/configure it for that specific layer.

**Describe the solution you'd like**
When configuring a probe, you'll want an option to type a layername (in addition to the dropdown).

**Describe alternatives you've considered**
Alternative: use SQL insert statements to configure the probe via the database and not via the front-end application.