Replace QScopedPointer with std::unqiue_ptr

These 2 types are very similar and we should prefer the one from C++
standard library, this reduces the number of types in our code base.

Also see:
https://stackoverflow.com/questions/40346393/should-i-use-qscopedpointer-or-stdunique-ptr#comment67966940_40346991

src/gui/fspathedit.h

@@ -105,7 +105,7 @@ private:
     Q_DISABLE_COPY(FileSystemPathEdit)
     class FileSystemPathEditPrivate;
     Q_DECLARE_PRIVATE(FileSystemPathEdit)
-    QScopedPointer<FileSystemPathEditPrivate> const d_ptr;
+    FileSystemPathEditPrivate *d_ptr;
 };
 
 /// Widget which uses QLineEdit for path editing