Trade: provide access to plugin manager in AbstractImageConverter.

Similarly to what is done in AbstractImporter, allows me to implement
AnyImageConverter plugin.

src/Magnum/Trade/AbstractImageConverter.cpp

@@ -35,7 +35,9 @@ namespace Magnum { namespace Trade {
 
 AbstractImageConverter::AbstractImageConverter() = default;
 
-AbstractImageConverter::AbstractImageConverter(PluginManager::AbstractManager& manager, std::string plugin): AbstractPlugin(manager, std::move(plugin)) {}
+AbstractImageConverter::AbstractImageConverter(PluginManager::Manager<AbstractImageConverter>& manager): PluginManager::AbstractManagingPlugin<AbstractImageConverter>{manager} {}
+
+AbstractImageConverter::AbstractImageConverter(PluginManager::AbstractManager& manager, std::string plugin): PluginManager::AbstractManagingPlugin<AbstractImageConverter>{manager, std::move(plugin)} {}
 
 std::optional<Image2D> AbstractImageConverter::exportToImage(const ImageView2D& image) const {
     CORRADE_ASSERT(features() & Feature::ConvertImage,

src/Magnum/Trade/AbstractImageConverter.h

@@ -29,7 +29,7 @@
  * @brief Class @ref Magnum::Trade::AbstractImageConverter
  */
 
-#include <Corrade/PluginManager/AbstractPlugin.h>
+#include <Corrade/PluginManager/AbstractManagingPlugin.h>
 
 #include "Magnum/Magnum.h"
 #include "Magnum/visibility.h"
@@ -64,7 +64,7 @@ checked by the implementation:
 
 Plugin interface string is `"cz.mosra.magnum.Trade.AbstractImageConverter/0.2.1"`.
 */
-class MAGNUM_EXPORT AbstractImageConverter: public PluginManager::AbstractPlugin {
+class MAGNUM_EXPORT AbstractImageConverter: public PluginManager::AbstractManagingPlugin<AbstractImageConverter> {
     CORRADE_PLUGIN_INTERFACE("cz.mosra.magnum.Trade.AbstractImageConverter/0.2.1")
 
     public:
@@ -110,6 +110,9 @@ class MAGNUM_EXPORT AbstractImageConverter: public PluginManager::AbstractPlugin
         /** @brief Default constructor */
         explicit AbstractImageConverter();
 
+        /** @brief Constructor with access to plugin manager */
+        explicit AbstractImageConverter(PluginManager::Manager<AbstractImageConverter>& manager);
+
         /** @brief Plugin manager constructor */
         explicit AbstractImageConverter(PluginManager::AbstractManager& manager, std::string plugin);