Incorrect ID references in layer control

[Malvoz]

Due to duplicate ids on the input elements and for="" id references in label elements one input in https://maps4html.org/experiments/datacube/ is labelled WMS GetMap twice and the other is not labelled.

(See "Name" under "Computed Properties" in the Accessibility tab in the screenshots)

[Malvoz]

With the latest changes from #531 I (just now, sorry) noticed some problems related to this issue:

• aria-labelledby must reference an ID (of the corresponding mapml-layer-item-name)
• id attributes must not contain whitespace