Replace "Master Key" with "Database Credentials"

Definitions:
* Database Key - Cryptographic hash used to perform encrypt/decrypt of the database.

* Database Credentials - User facing term to refer to the collection of Password, Key File, and/or Hardware Key used to derive the Database Key.

Changes:
* Remove the term "master" and "key" from the user's lexicon and clarify  the code base based on the definitions above.
* Clean up wording in the UI to be clearer to the end user.

src/format/KeePass1Reader.cpp

@@ -242,7 +242,7 @@ KeePass1Reader::readDatabase(QIODevice* device, const QString& password, QIODevi
     }
 
     if (!db->setKey(key)) {
-        raiseError(tr("Unable to calculate master key"));
+        raiseError(tr("Unable to calculate database key"));
         return {};
     }