FIX: use thread stack to keep shared ref of PrinterFileSystem

Change-Id: I86dce263f43eb65074d4fec777cb08885505b7cf
(cherry picked from commit 1dbc32b2335c33831ac94a787c8e7ec7d898ccba)

src/slic3r/GUI/Printer/PrinterFileSystem.h

@@ -149,6 +149,8 @@ public:
     Status GetStatus() const { return m_status; }
     int GetLastError() const { return m_last_error; }
 
+    void Attached();
+
     void Start();
 
     void Retry();