vlp: Automatically reduce separate files and prefer directories.

 * Try to avoid <foobar-files> and use <foobar-directories> instead to shorten
   project file.
 * Fix get_relative_path null issue.
 * Use private _foo fields because ref is not allowed with properties.
Generate dirs/files to save (automatically) only before saving.

 * Add lots of data files to .vlp.
   - Currently there is no check for valid plaintext files.
 * Allow creation of (sub)directories with ui_create_project (allow / and \).
 * Recursive directory lookup would be nice and shorten .vlp.