Use component id in class object of model JSON

This change is needed to work with the API. Also update comments in
other classes.
Add null check to ModelTree#isComponentPresent
Implement "New Model" and "Close Model" menu items

"New Model" creates a new instance of WMT in a new browser tab. Added
DataURL#getApplicationURL to assist.

"Close Model" resets the WMT GUI in the current tab. Added
Perspective#reset to assist.