Begin implementation of Screen Switching from the Companion.

GWT Coding complete (and tested via browser debugger). Blockly and
Companion code still to do...

Also cleaned up the “indicateDisconnected” code in
DesignerToolbar. Use existing method of finding the non-static
DesignerToolbar object instead of code I added which duplicated that
behavior.

Change-Id: I490cb16966018e8aaf1477d53d66ba035d423bcc