Plugin: typedef DescriptorList + more const improvements

Instead of writing QVector<Plugin::Descriptor> everywhere, mix up
iterators etc. centrally define Plugin::DescriptorList and use it
everywhere.

Additionally made pointers to Plugin::Descriptor and
Plugin::Descriptor::SubPluginFeatures::Key const in various places
so we can use ConstIterator everywhere when iterating through
Plugin::DescriptorList.
(cherry picked from commit 2287eca802)

src/gui/EffectSelectDialog.cpp

@@ -44,7 +44,7 @@ EffectSelectDialog::EffectSelectDialog( QWidget * _parent ) :
 	// query effects
 	Plugin::getDescriptorsOfAvailPlugins( m_pluginDescriptors );
 
-	for( QVector<Plugin::Descriptor>::Iterator it = m_pluginDescriptors.begin();
+	for( Plugin::DescriptorList::ConstIterator it = m_pluginDescriptors.begin();
 										it != m_pluginDescriptors.end(); ++it )
 	{
 		if( it->type != Plugin::Effect )