Platform: display disabled extension status in magnum-info.

Currently named as "removed" until I find a word which is *even* amount
of characters shorter than "supported" ("disabled" is not and it looks
ugly). Also marked extensions that can't be supported in current GL
version with "n/a" instead of "---".

src/Magnum/Platform/magnum-info.cpp

@@ -184,10 +184,12 @@ MagnumInfo::MagnumInfo(const Arguments& arguments): Platform::WindowlessApplicat
             d << "   " << extensionName << std::string(60-extensionName.size(), ' ');
             if(c->isExtensionSupported(extension))
                 d << "SUPPORTED";
+            else if(c->isExtensionDisabled(extension))
+                d << " removed";
             else if(c->isVersionSupported(extension.requiredVersion()))
                 d << "    -";
             else
-                d << "   ---";
+                d << "   n/a";
         }
 
         Debug() << "";