ProjectExplorer: Introduce and use a Target::buildSystemUpdated signal

Change-Id: I497d46866146600a1ed9162bd720c574ee4d4769
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/nim/project/nimbuildsystem.cpp

@@ -160,6 +160,8 @@ void NimBuildSystem::updateProject()
     // Complete scan
     m_guard.markAsSuccess();
     m_guard = {}; // Trigger destructor of previous object, emitting parsingFinished()
+
+    emitBuildSystemUpdated();
 }
 
 bool NimBuildSystem::supportsAction(Node *context, ProjectAction action, const Node *node) const