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/core/PasswordGenerator.cpp

@@ -31,7 +31,7 @@ PasswordGenerator::PasswordGenerator()
 {
 }
 
-double PasswordGenerator::calculateEntropy(QString password)
+double PasswordGenerator::calculateEntropy(const QString& password)
 {
     return ZxcvbnMatch(password.toLatin1(), 0, 0);
 }