Allow user to set default theme

During theme switch the user can now set the selected theme as default
This setting is saved as configuration scope and loaded for new
workspaces unless the user has a theme set for this specific workspace.
Add 'Use as default theme' checkbox to theme
restart dialog

Author: vogella

bundles/org.eclipse.e4.ui.css.swt.theme/src/org/eclipse/e4/ui/css/swt/internal/theme/ThemeEngine.java

@@ -40,6 +40,7 @@
 import org.eclipse.core.runtime.IPath;
 import org.eclipse.core.runtime.Platform;
 import org.eclipse.core.runtime.RegistryFactory;
+import org.eclipse.core.runtime.preferences.ConfigurationScope;
 import org.eclipse.core.runtime.preferences.IEclipsePreferences;
 import org.eclipse.core.runtime.preferences.IPreferencesService;
 import org.eclipse.core.runtime.preferences.InstanceScope;