Allow optional plugin dependencies.

Adds a 'type' attribute to the 'dependency' tag, with possible values
'required' and 'optional'.
Optional dependencies may not be linked against. You'll need to use the
new dynamic methods in plugin manager (retrieving objects and calling
methods by name) if you want to access functionality of optional
dependencies.

src/libs/extensionsystem/plugindetailsview.cpp

@@ -95,6 +95,8 @@ void PluginDetailsView::update(PluginSpec *spec)
         QString depString = dep.name;
         depString += QLatin1String(" (");
         depString += dep.version;
+        if (dep.type == PluginDependency::Optional)
+            depString += QLatin1String(", optional");
         depString += QLatin1Char(')');
         depStrings.append(depString);
     }