Fix DLT file append failure and resolve path encoding for non-Latin1 characters characters

- Fix file opening issues in environments with non-Latin1 characters (e.g., Chinese) by using toLocal8Bit() instead of toLatin1() for dlt_file_open calls.
- Fix on_tabExplore_filesAppendRequest to correctly handle appending .dlt files by invoking appendDltFile.
- Add support for appending .dlf filter files and improve handling for PCAP/MF4 files in the file explorer's append request.
- Refactor file open and append logic in MainWindow for better consistency and broader file type support.

@shubhamshahaBMW I have fixed the vulnerability related to appending files and verified the fix in a Linux environment. During the process, I discovered that calling dlt_open_file fails in non-Latin (e.g., Chinese) environments. Regarding the "show in explorer" failure mentioned in https://github.com/COVESA/dlt-viewer/pull/779, I was unable to reproduce it locally; currently, "show in explorer" only works when a single file or folder is selected.
Could you please help me check if it works on your side?

#790 this is the pull requests.