PTService: add utility method to create a plugin

Surprisingly, the PluginService API does not have a method to create a
plugin from its class. You can only create one from its associated
PluginInfo. So instantiating a plugin becomes a two-step process:

1. Ask the PluginService for the PluginInfo of a given plugin Class.
2. Tell the PluginService to create an instance from that PluginInfo.

And this process is not fully type-safe, since the
PluginInfo#createInstance(PluginInfo) method only knows the object is
of type PT, not P.

To help address this limitation, each PTService now has a handy
create(Class<P extends PT>) method for creating an instance of that
plugin class. This is useful, since using "new" directly is not enough;
the plugin will not have the application context injected as needed.

Author: ctrueden

src/main/java/org/scijava/plugin/AbstractPTService.java

@@ -60,4 +60,13 @@ public List<PluginInfo<PT>> getPlugins() {
 		return pluginService.getPluginsOfType(getPluginType());
 	}
 
+	@Override
+	public <P extends PT> P create(final Class<P> pluginClass) {
+		final PluginInfo<PT> info =
+			pluginService.getPlugin(pluginClass, getPluginType());
+		@SuppressWarnings("unchecked")
+		final P plugin = (P) pluginService.createInstance(info);
+		return plugin;
+	}
+
 }

src/main/java/org/scijava/plugin/PTService.java

@@ -95,4 +95,7 @@ public interface PTService<PT extends SciJavaPlugin> extends Service {
 	/** Gets the type of plugins managed by this service. */
 	Class<PT> getPluginType();
 
+	/** Creates an instance of the given plugin class. */
+	<P extends PT> P create(final Class<P> pluginClass);
+
 }