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/TestPassphraseGenerator.cpp

@@ -16,6 +16,7 @@
  */
 
 #include "TestPassphraseGenerator.h"
+#include "config-keepassx-tests.h"
 #include "core/PassphraseGenerator.h"
 #include "crypto/Crypto.h"
 
@@ -52,3 +53,18 @@ void TestPassphraseGenerator::testWordCase()
     QRegularExpression regex("^(?:[A-Z][a-z-]* )*[A-Z][a-z-]*$");
     QVERIFY2(regex.match(passphrase).hasMatch(), qPrintable(passphrase));
 }
+
+void TestPassphraseGenerator::testUniqueEntriesInWordlist()
+{
+    PassphraseGenerator generator;
+    // set the limit down, so we don;t have to do a very large file
+    generator.m_minimum_wordlist_length = 4;
+
+    // link to bad wordlist
+    QString path = QString(KEEPASSX_TEST_DATA_DIR).append("/wordlists/bad_wordlist_with_duplicate_entries.wordlist");
+
+    // setting will work, it creates the warning however, and isValid will fail
+    generator.setWordList(path);
+    // so this fails
+    QVERIFY(!generator.isValid());
+}