Trade: don't define AbstractImporter constructors in header.

They are heavier than it seems due to inheritance and virtual
functions.

src/Trade/AbstractImporter.cpp

@@ -30,6 +30,10 @@
 
 namespace Magnum { namespace Trade {
 
+AbstractImporter::AbstractImporter() = default;
+
+AbstractImporter::AbstractImporter(Corrade::PluginManager::AbstractPluginManager* manager, std::string plugin): AbstractPlugin(manager, std::move(plugin)) {}
+
 bool AbstractImporter::open(const std::string&) {
     Error() << plugin() << "doesn't support opening files";
     return false;

src/Trade/AbstractImporter.h

@@ -71,10 +71,10 @@ class MAGNUM_EXPORT AbstractImporter: public Corrade::PluginManager::AbstractPlu
         typedef Corrade::Containers::EnumSet<Feature, int> Features;
 
         /** @brief Default constructor */
-        inline explicit AbstractImporter() = default;
+        explicit AbstractImporter();
 
         /** @brief Plugin manager constructor */
-        inline explicit AbstractImporter(Corrade::PluginManager::AbstractPluginManager* manager, std::string plugin): AbstractPlugin(manager, std::move(plugin)) {}
+        explicit AbstractImporter(Corrade::PluginManager::AbstractPluginManager* manager, std::string plugin);
 
         /** @brief Features supported by this importer */
         virtual Features features() const = 0;