A new class QgsPathResolver is introduced for conversion between absolute
and relative paths when reading/writing XML.
Cleaned up code in layer definition file support (.qlr) to better handle
relative/absolute path conversion.
Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!
