KPropertiesDialog: fix saving changes when editing a .desktop file symlink

Consider the use case of:
/usr/share/applications/foo.desktop is symlink to /opt/someapp/foo.desktop

relativeAppsLocation() was using QFile::canonicalFilePath(), which meant
that foo.desktop would resolve to a file outside /usr/share/applications,
since the code, rightly, expects files in QStandardPaths::ApplicationsLocation
locations, m_sRelativePath would end up being empty, which makes the check in
KFilePropsPlugin::slotCopyFinished() fail, and the .desktop file wouldn't
get copied to ~/.local/share/applications/, and the saving would fail.

BUG: 450727
FIXED-IN: 5.96