MagnumFont,MagnumFontConverter: adapt to Corrade changes.

And document this in MagnumFont and MagnumFontConverter usage docs. The
inter-manager dependencies weren't an unused feature after all.

doc/snippets/plugins.cpp

@@ -2,6 +2,8 @@
 #include <Corrade/Utility/ConfigurationGroup.h>
 
 #include "Magnum/Text/AbstractFont.h"
+#include "Magnum/Text/AbstractFontConverter.h"
+#include "Magnum/Trade/AbstractImageConverter.h"
 #include "Magnum/Trade/AbstractImporter.h"
 
 #ifdef __has_include
@@ -68,4 +70,20 @@ importer->configuration().group("postprocess")->setValue("PreTransformVertices",
 /* [configuration] */
 }
 
+{
+/* [MagnumFont-importer-register] */
+PluginManager::Manager<Trade::AbstractImporter> importerManager;
+PluginManager::Manager<Text::AbstractFont> fontManager;
+fontManager.registerExternalManager(importerManager);
+/* [MagnumFont-importer-register] */
+}
+
+{
+/* [MagnumFontConverter-imageconverter-register] */
+PluginManager::Manager<Trade::AbstractImageConverter> imageConverterManager;
+PluginManager::Manager<Text::AbstractFontConverter> fontConverterManager;
+fontConverterManager.registerExternalManager(imageConverterManager);
+/* [MagnumFontConverter-imageconverter-register] */
+}
+
 }

src/MagnumPlugins/MagnumFont/MagnumFont.h

@@ -144,6 +144,12 @@ find_package(Magnum REQUIRED MagnumFont)
 target_link_libraries(your-app PRIVATE Magnum::MagnumFont)
 @endcode
 
+Because the plugin needs access to @ref Trade::AbstractImporter plugins, you
+need to instantiate a manager for them and register it with
+@ref Corrade::PluginManager::Manager::registerExternalManager():
+
+@snippet plugins.cpp MagnumFont-importer-register
+
 See @ref building, @ref cmake and @ref plugins for more information.
 */
 class MAGNUM_MAGNUMFONT_EXPORT MagnumFont: public AbstractFont {

src/MagnumPlugins/MagnumFont/Test/MagnumFontGLTest.cpp

@@ -50,6 +50,7 @@ MagnumFontGLTest::MagnumFontGLTest() {
 
     /* Load the plugins directly from the build tree. Otherwise they're static
        and already loaded. */
+    _fontManager.registerExternalManager(_importerManager);
     #if defined(TGAIMPORTER_PLUGIN_FILENAME) && defined(MAGNUMFONT_PLUGIN_FILENAME)
     CORRADE_INTERNAL_ASSERT_OUTPUT(_importerManager.load(TGAIMPORTER_PLUGIN_FILENAME) & PluginManager::LoadState::Loaded);
     CORRADE_INTERNAL_ASSERT_OUTPUT(_fontManager.load(MAGNUMFONT_PLUGIN_FILENAME) & PluginManager::LoadState::Loaded);

src/MagnumPlugins/MagnumFont/Test/MagnumFontTest.cpp

@@ -64,6 +64,7 @@ MagnumFontTest::MagnumFontTest() {
 
     /* Load the plugins directly from the build tree. Otherwise they're static
        and already loaded. */
+    _fontManager.registerExternalManager(_importerManager);
     #if defined(TGAIMPORTER_PLUGIN_FILENAME) && defined(MAGNUMFONT_PLUGIN_FILENAME)
     CORRADE_INTERNAL_ASSERT_OUTPUT(_importerManager.load(TGAIMPORTER_PLUGIN_FILENAME) & PluginManager::LoadState::Loaded);
     CORRADE_INTERNAL_ASSERT_OUTPUT(_fontManager.load(MAGNUMFONT_PLUGIN_FILENAME) & PluginManager::LoadState::Loaded);

src/MagnumPlugins/MagnumFontConverter/MagnumFontConverter.h

@@ -87,6 +87,12 @@ find_package(Magnum REQUIRED MagnumFontConverter)
 target_link_libraries(your-app PRIVATE Magnum::MagnumFontConverter)
 @endcode
 
+Because the plugin needs access to @ref Trade::AbstractImageConverter plugins,
+you need to instantiate a manager for them and register it with
+@ref Corrade::PluginManager::Manager::registerExternalManager():
+
+@snippet plugins.cpp MagnumFontConverter-imageconverter-register
+
 See @ref building, @ref cmake and @ref plugins for more information.
 */
 class MAGNUM_MAGNUMFONTCONVERTER_EXPORT MagnumFontConverter: public Text::AbstractFontConverter {

src/MagnumPlugins/MagnumFontConverter/Test/MagnumFontConverterTest.cpp

@@ -61,6 +61,7 @@ MagnumFontConverterTest::MagnumFontConverterTest() {
 
     /* Load the plugins directly from the build tree. Otherwise they are static
        and already loaded. */
+    _fontConverterManager.registerExternalManager(_imageConverterManager);
     #if defined(TGAIMAGECONVERTER_PLUGIN_FILENAME) && defined(MAGNUMFONTCONVERTER_PLUGIN_FILENAME)
     CORRADE_INTERNAL_ASSERT_OUTPUT(_imageConverterManager.load(TGAIMAGECONVERTER_PLUGIN_FILENAME) & PluginManager::LoadState::Loaded);
     CORRADE_INTERNAL_ASSERT_OUTPUT(_fontConverterManager.load(MAGNUMFONTCONVERTER_PLUGIN_FILENAME) & PluginManager::LoadState::Loaded);