Fix crashes on database save

* Add saving mutex to database class to prevent re-entrant saving
* Prevent saving multiple times to the same file if the database is not marked as modified
* Prevent locking the database while saving. This also prevents closing the application and database tab while saving.
* FileWatcher: only perform async checksum calculations when triggered by timer (prevents random GUI freezes)
* Re-attempt database lock when requested during save operation
* Prevent database tabs from closing before all databases are locked on quit

src/cli/Create.cpp

@@ -121,6 +121,7 @@ int Create::execute(const QStringList& arguments)
 
     QSharedPointer<Database> db(new Database);
     db->setKey(key);
+    db->setInitialized(true);
 
     if (decryptionTime != 0) {
         auto kdf = db->kdf();