Fix #1605, Sandcastle drop-downs don't work in Firefox
Fix #1603

#1603 only affected non-time-varying data sources.  After the last revamp of the Property system,  `DataSource.getIsTimeVarying` is actually no longer needed and removing it fixes a whole host of issues similar to #1603.
Merge pull request #1615 from AnalyticalGraphicsInc/sandcastle-select

Fix #1605, Sandcastle drop-downs don't work in Firefox
Merge pull request #1619 from AnalyticalGraphicsInc/issue1603

Fix #1603
Merge branch 'master' into dynamicScene-buffering

Conflicts:
	Source/DynamicScene/DataSourceDisplay.js