Scan Folder dialog now remembers last location

When you add a new scan folder location it will now remember the last location used so you can add a bunch of folders in a row easily.
I didn't think it was important to name every variable exactly as you wrote.

I am naming the function to geScanDirsLastPath/setScanDirsLastPath since that is the backing vars name.

Fixing the previous one to have get in front of it. I followed the convention of 
````
  QString lastLocationPath() const {
    return fsutils::fromNativePath(value(QString::fromUtf8("Preferences/Downloads/LastLocationPath"), QString()).toString());
  }

  void setLastLocationPath(const QString &path) {
    setValue(QString::fromUtf8("Preferences/Downloads/LastLocationPath"), fsutils::fromNativePath(path));
  }
````