Make FullscreenViewModel accept element id

Everywhere in Cesium we take an element, we also allow the string id to
be used instead. This just updates `FullscreenViewModel` to be in line with
everything else,
Merge pull request #2339 from AnalyticalGraphicsInc/fullscreen-element

Make FullscreenViewModel accept element id
Merge remote-tracking branch 'origin/master' into implicit-properties