Use labels on custom layers configuration page, not keys

Problem/Motivation

Currently, the machine name-like keys are used to select maps under the custom layers, such as "Custom map #1 layer selection" at /admin/config/system/leaflet-more-maps.

For example:

[...]

• mapycz-winter layer
• mapycz-aerial layer
• osm-de layer
• osm-cycle layer
• osm-transport layer

[...]

This is not ideal, if for example you type "open" and expect openstreetmap to be found, but get no result. It also looks visually more appealing and less machine-like, if human readable map names are used.

Proposed resolution

We could use the labels instead:

Now
osm-de layer

Better?
OpenStreetMap.de (zoom 0..18) layer (osm-de)

Remaining tasks

Update src/Form/SettingsForm.php to use the label for the different maps in the Custom layer lists, instead of the key.

User interface changes

API changes

Data model changes