WIP PerformanceWatchdog.
Merge remote-tracking branch 'origin/master' into performanceMonitorWidget
Merge branch 'widgetAndViewerImprovements' into performanceMonitorWidget

Conflicts:
	Source/Widgets/CesiumWidget/CesiumWidget.js
	Source/Widgets/Viewer/Viewer.js
Fix bad merge.
Mostly-working watchdog widget.
Hide the message when the framerate improves.
Don't measure performance while the window is hidden.
Merge remote-tracking branch 'origin/master' into performanceMonitorWidget
Error handling improvements.
Make low frame rate message dismissable.
Incomplete doc and tests.
Merge remote-tracking branch 'origin/master' into performanceMonitorWidget

Conflicts:
	Source/Widgets/Viewer/Viewer.js
Proper view model destruction, and more tests.
Make the test more robust.
More tests, and remove some dead code.
Add tests for PerformanceWatchdog widget itself.
Merge remote-tracking branch 'origin/master' into performanceMonitorWidget
Improve styling of low frame rate message.
Factor frame rate monitoring out of the Viewer widget.
Merge remote-tracking branch 'origin/master' into performanceMonitorWidget