Ferret query action should do less on UI thread

[briandealwis]

ca.ubc.cs.ferret.AskFerretWorkbenchAction calls getSelectedObjects() on the sphere helpers on the UI thread, which may lead to a lot of computation.

!SUBENTRY 1 org.eclipse.ui.monitoring 1 0 2017-06-01 11:53:44.613
!MESSAGE Sample at 11:53:44.371 (+0.328s)
Thread 'main' tid=1 (RUNNABLE)
!STACK 0
Stack Trace
	at java.io.UnixFileSystem.$$YJP$$getBooleanAttributes0(Native Method)
	at java.io.UnixFileSystem.getBooleanAttributes0(UnixFileSystem.java)
	at java.io.UnixFileSystem.getBooleanAttributes(UnixFileSystem.java:242)
	at java.io.File.exists(File.java:819)
	at org.eclipse.pde.internal.core.ExternalFeatureModelManager.getExternalModels(ExternalFeatureModelManager.java:126)
	at org.eclipse.pde.internal.core.ExternalFeatureModelManager.initialize(ExternalFeatureModelManager.java:85)
	at org.eclipse.pde.internal.core.FeatureModelManager.init(FeatureModelManager.java:119)
	at org.eclipse.pde.internal.core.FeatureModelManager.getExternalModels(FeatureModelManager.java:150)
	at ca.ubc.cs.ferret.pde.PdeModelHelper.verifyModelCaches(PdeModelHelper.java:226)
	at ca.ubc.cs.ferret.pde.PdeModelHelper.findPluginModel(PdeModelHelper.java:240)
	at ca.ubc.cs.ferret.pde.PdeSphereHelper.getSelectedObjects(PdeSphereHelper.java:127)
	at ca.ubc.cs.ferret.pde.PdeSphereHelper.getSelectedObjects(PdeSphereHelper.java:115)
	at ca.ubc.cs.ferret.AskFerretWorkbenchAction$1.run(AskFerretWorkbenchAction.java:48)
	at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:37)
	at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:182)
	at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4033)
	at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3700)
	at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$5.run(PartRenderingEngine.java:1155)