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/Import.cpp

@@ -84,6 +84,7 @@ int Import::execute(const QStringList& arguments)
     Database db;
     db.setKdf(KeePass2::uuidToKdf(KeePass2::KDF_ARGON2));
     db.setKey(key);
+    db.setInitialized(true);
 
     if (!db.import(xmlExportPath, &errorMessage)) {
         errorTextStream << QObject::tr("Unable to import XML database: %1").arg(errorMessage) << endl;