FileSaver: Don't add files in blacklisted paths

We now resolve symlinks when adding, so we can properly check
whether a path is excluded or not. This accidently also
helps with duplicates.

Excluded paths are hardcoded and can also be appended to
by the user using the settings.

Closes: #34

shared/common.h

@@ -11,5 +11,6 @@ void setPdfViewer();
 QString findInPath(QString needle);
 bool initSqliteDatabase(QString path);
 void ensureConfigured();
+QStringList excludedPaths();
 } // namespace Common
 #endif