Fixed epub generator not being able to load resources on some files.

Cases that are fixed by this change are:
- Filenames that didn't start with "/" weren't concatenated properly to
  the base path.
- Non-normalized paths (eg: having "../") couldn't be resolved by libepub.
  QUrl::resolved() normalizes the  paths after concatenating them.
- Paths with special characters were escaped because of using
  QUrl::toString() instead of QUrl::path(), making them not work.

Reviewed by Albert Astals.