Fix broken split screen

The Id attribute was used in the ESRI sample, but in addition to
registering a DOM attribute (ID), it also registers and ID in a
weird global registry that doesn't allow duplicates, and was silently
failing.  Use a class instead, which is better markup since there
could be two elements on the screen at once (split screen) and
doesn't have a registry conflict.
Use relative paths for loading resources

This allows the plugin to be moved to other folders or copied and
still function correctly.
Merge pull request #215 from CoastalResilienceNetwork/bugfix/split-screen-broke

Fix split screen and more