Improve posix support

[MHendricks]

• Use xdg-open to browse files on linux
• Fix pref_updates.json path to work on linux
• Fix issue trying to join a PosixPath with None

I can't easily replicate this issue, it seems to be related to prefs migration, but this is the traceback I debugged and the code was able to fix a instance of it.

Traceback (most recent call last):
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/gui/errordialog.py", line 58, in show_logger
    launch()
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/__init__.py", line 175, in launch
    widget = LoggerWindow.instance(
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/gui/loggerwindow.py", line 2306, in instance
    inst = LoggerWindow(
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/gui/loggerwindow.py", line 154, in __init__
    self.restorePrefs()
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/gui/loggerwindow.py", line 1652, in restorePrefs
    self.restoreWorkboxPrefs(pref)
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/gui/loggerwindow.py", line 1624, in restoreWorkboxPrefs
    self.uiWorkboxTAB.restore_prefs(workbox_prefs)
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/gui/group_tab_widget/group_tab_widget.py", line 382, in restore_prefs
    backup_file, _, count = get_backup_version_info(
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/prefs.py", line 262, in get_backup_version_info
    files = get_file_group(core_name, workbox_id)
  File "/usr/local/venv/py39qt5152/20665/python/lib/python3.9/site-packages/preditor/prefs.py", line 209, in get_file_group
    workbox_dir = directory / workbox_id
TypeError: unsupported operand type(s) for /: 'PosixPath' and 'NoneType'

The old preditor_pref.json"

    "workbox_prefs": {
        "groups": [
            {
                "current": true,
                "name": "Group01",
                "tabs": [
                    {
                        "current": true,
                        "filename": null,
                        "name": "Workbox01",
                        "tempfile": "workbox_4pmd088k.py"
                    }
                ]
            }
        ]
    }

The fixed preditor_pref.json" after this fix is applied.

    "workbox_prefs": {
        "groups": [
            {
                "current": true,
                "name": "Group01",
                "tabs": [
                    {
                        "backup_file": "workbox_4pmd088k/workbox_4pmd088k-2026-01-20-17-51-19-616951.py",
                        "current": true,
                        "filename": null,
                        "name": "Workbox01",
                        "tempfile": "workbox_4pmd088k.py",
                        "workbox_id": "workbox_4pmd088k"
                    }
                ]
            }
        ]
    }

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)