Added highlight feature to indicate layers can be changed.

Layers are now highlighted when hovered over, providing the user with a clear indicator that 'something' can be done when there. Visited link styles are also brought back in line with regular links, preventing the dark purple default.
Fixed layer click event propagation, cursor indication and click area size.
Handle floating point `steps` argument by converting it to int.
Moved api paths to library-global variables.
Apply consistent HTML class/ID naming.

All classes and IDs in HTML are now named in lowercase with dashes to separate words.