Option to save with absolute paths

### Enhancement Summary

A checkbox in the Save Options section of the Save As menu like the ones for *Discard MIDI connections* and *Save as Project Bundle (with resources)* would make LMMS convert all relative paths to absolute in the saved project file.

#### Justification

Converting projects from relative to absolute is tedious for a user, but easy for LMMS. This would for example allow users to save projects in a format more compatible with 1.2 and earlier versions, or prevent them from breaking if they intend to change their LMMS folder settings without moving the folder contents in an equivalent way.