Reduce number of unneeded copies

This patch aims at reducing the number of copies for obejcts that could
be referenced rather than copied, because they're not modified during
the computation.

src/cli/Clip.cpp

@@ -71,7 +71,7 @@ int Clip::execute(const QStringList& arguments)
     return clipEntry(db, args.at(1), args.value(2));
 }
 
-int Clip::clipEntry(Database* database, QString entryPath, QString timeout)
+int Clip::clipEntry(Database* database, const QString& entryPath, const QString& timeout)
 {
     TextStream err(Utils::STDERR);