The playlist export dialog should support using relative paths for exported playlists. This is necessary when creating playlist files that will be used with the same files but on other OSes, computers, or devices.
Offhand I can think of two ways to implement the user interface for this:
1) The export dialog gains a file chooser button to set the directory to export relative to, which will default to / (thus avoiding any special-casing). If the user attempts to set that path to a path not shared by all files in the current playlist, an error is displayed and the user is sent back into the dialog for choosing the relative path.
2) Instead of a full file chooser, auto-detect the common path elements and provide a means of selecting among them for them - this could be more intuitive and easier to use, but would be harder to implement and if the shared paths are really long could be awkward to display. I think this is the better option if we can handle it sanely however.
Relatedly, we should probably support relative paths for import as well, prompting the user for a path when a playlist using relative paths is imported. Not much use to be able to export relative paths if we can't use the resulting files. :)