Prevent duplicate entries in passphrase wordlists

Replace a QVector for the wordlist with a QSet. This removes all duplicate entries in a given wordlist.
Thus, it hinders a malicious wordlist that has the proper length (>4000 entries) but with repetitions (effectively << 4000 entries) to be used and potentially create weaker passphrases than estimated.

Example:
List with 4000 items but only 64 unique words would lead to only 48 bit of Entropy instead of ~95 bit!

tests/TestCli.cpp

@@ -1088,7 +1088,7 @@ void TestCli::testDiceware()
     smallWordFile.close();
 
     execCmd(dicewareCmd, {"diceware", "-W", "11", "-w", smallWordFile.fileName()});
-    QCOMPARE(m_stderr->readLine(), QByteArray("The word list is too small (< 1000 items)\n"));
+    QCOMPARE(m_stderr->readLine(), QByteArray("Cannot generate valid passphrases because the wordlist is too short\n"));
 }
 
 void TestCli::testEdit()