Move long running operations to the activation of the PluginsTab

Contributes to eclipse-pde#1232

ui/org.eclipse.pde.ui/src/org/eclipse/pde/ui/launcher/PluginsTab.java

@@ -59,6 +59,7 @@ public class PluginsTab extends AbstractLauncherTab {
 	private Combo fDefaultAutoStart;
 	private Spinner fDefaultStartLevel;
 	private final Listener fListener;
+	private boolean fActivated;
 
 	private static final int DEFAULT_SELECTION = 0;
 	private static final int PLUGIN_SELECTION = 1;
@@ -156,6 +157,16 @@ public void createControl(Composite parent) {
 
 	@Override
 	public void initializeFrom(ILaunchConfiguration configuration) {
+		// Long-running operations happen during the first activation of the tab
+	}
+
+	@Override
+	public void activated(ILaunchConfigurationWorkingCopy configuration) {
+		if (fActivated) {
+			// Since this method can be expensive, only activate this tab once.
+			return;
+		}
+
 		try {
 			int index = DEFAULT_SELECTION;
 			if (configuration.getAttribute(IPDELauncherConstants.USE_CUSTOM_FEATURES, false)) {
@@ -174,6 +185,8 @@ public void initializeFrom(ILaunchConfiguration configuration) {
 		} catch (CoreException e) {
 			PDEPlugin.log(e);
 		}
+
+		fActivated = true;
 	}
 
 	@Override