KgThemeProvider: fix generating broken full path theme id due to fs symlinks
relativeToApplications() was using QFileInfo(file).canonicalFilePath() to compare against non-canonicalized directories returned by QStandardPaths, thus missing out in case the file was in a path with symlinks, resulting in ids with full absolute path.
This patch fixes that by calculating the id from the known one subdir name and the desktop file name directly.
Patch so far based on theory as commented in the bug report.