CommandDelete: Rework deletion logic

Allow deleting files from index which still exist
on the fileystem without passing their path to "qss delete".

Thus: "qss delete --deleted" deletes all files which
don't exist anymore.

Also, fix some bugs in offset calculation.

cli/commanddelete.h

@@ -10,7 +10,7 @@ class CommandDelete : public Command
 	int handle(QStringList arguments) override;
 
   private:
-	int removeNonExistent(bool verbose, bool dryRun, QString pattern);
+	int remove(QString pattern, bool onlyDeleted, bool verbose, bool dryRun);
 	int removePaths(const QStringList &paths, bool verbose, bool dryRun);
 };