[Feature] app.fs.lastSavedPath

As the title suggests, when saving an image using the dialog file, it doesn't include a path by default if you use filename = "test.png" for example, so dialog:file will return "test.png" by default and thus you will be saving to app.fs.currentPath/test.png.

Having a app.fs.lastSavedPath will at least default to a nice location. However one can use app.fs.userDocsPath for now.