[wpigui] Fix PFD file dialogs not closing after window closing (#5530)

Also applies samhocevar/portable-file-dialogs#91 to properly retrieve the HWND on Windows.

wpigui/src/main/native/include/portable-file-dialogs.h

@@ -128,6 +128,7 @@ class executor;
 class dialog : protected settings
 {
 public:
+    virtual ~dialog();
     bool ready(int timeout = default_wait_timeout) const;
     bool kill() const;
 
@@ -146,7 +147,7 @@ protected:
     std::shared_ptr<executor> m_async;
 };
 
-class file_dialog : public dialog
+class file_dialog : public internal::dialog
 {
 protected:
     enum type