Refactor Database and Database widgets (#2491)

The Database, DatabaseWidget, and DatabaseTabWidget classes share many responsibilities in inconsistent ways resulting in impenetrable and unmaintainable code and a diverse set of bugs and architecture restrictions. This patch reworks the architecture, responsibilities of, and dependencies between these classes.

The core changes are:

* Move loading and saving logic from widgets into the Database class
* Get rid of the DatabaseManagerStruct and move all the information contained in it into the Database
* Let database objects keep track of modifications and dirty/clean state instead of handing this to external widgets
* Move GUI interactions for loading and saving from the DatabaseTabWidget into the DatabaseWidget (resolves #2494 as a side-effect)
* Heavily clean up DatabaseTabWidget and degrade it to a slightly glorified QTabWidget
* Use QSharedPointers for all Database objects
* Remove the modifiedImmediate signal and replace it with a markAsModified() method
* Implement proper tabName() method instead of reading back titles from GUI widgets (resolves #1389 and its duplicates #2146 #855)
* Fix unwanted AES-KDF downgrade if database uses Argon2 and has CustomData
* Improve code

This patch is also the first major step towards solving issues #476 and #2322.

src/keys/drivers/YubiKey.cpp

@@ -18,8 +18,6 @@
 
 #include <stdio.h>
 
-#include <QDebug>
-
 #include <ykcore.h>
 #include <ykdef.h>
 #include <ykstatus.h>
@@ -215,9 +213,9 @@ YubiKey::ChallengeResult YubiKey::challenge(int slot, bool mayBlock, const QByte
              */
 
             if (yk_errno == YK_EUSBERR) {
-                qWarning() << "USB error:" << yk_usb_strerror();
+                qWarning("USB error: %s",  yk_usb_strerror());
             } else {
-                qWarning() << "YubiKey core error:" << yk_strerror(yk_errno);
+                qWarning("YubiKey core error: %s", yk_strerror(yk_errno));
             }
 
             return ERROR;