Fix Open and Save Playlist global menu entries

Before, the two entries "Open Playlist…" and "Save Playlist…" in the "File" menu were bound to the same action as "Refresh Music Collection" which is obviously not wanted.

I merely copied and pasted the relevant parts from MediaPlayListView.qml and I'm not sure if it's ideal to have that PlatformDialog.FileDialog part at the bottom duplicated in both files but it works and I don't know how to do it otherwise.