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
Reset map layers on hibernate

Client requests that plugins reset their state when closed and
reopened.  Map layers previously held onto state so it could
be recreated easily, but this is not the desired behavior.
Merge pull request #216 from mmcfarland/topic/reset-on-close

Reset map layers on hibernate