GUI: Fix redundant signal calls, streamline path selections (#3395)

* Lower GUI video logs to debug

Change two MainWindow log calls in `deeplabcut/gui/window.py` from info to debug when setting the video type and clearing video files. This reduces routine GUI noise in normal logs while keeping the messages available for troubleshooting.

* Guard Top-Down method check in training tab

Hardened the GUI logic that toggles detector snapshot selection by safely handling missing `pose_cfg` or `method` values. The check now defaults to an empty dict/string before lowercasing, preventing runtime errors and preserving the intended Top-Down (`td`) behavior.

* Fix file selection and state sync

- Call QFileDialog positionally for PySide6 compatibility
- Normalize Path values before passing them to Qt
- Handle single-file and multi-file dialog results correctly
- Preserve Path objects in GUI state
- Prevent video type and shuffle signal feedback loops casing slow and redundant updates
- Retain ConfigEditor instances after opening

* use os.fspath instead of str

* revert use os.fspath (d795bab96d434f3f542d8316a3b41511b292be1b)

str() conversions are clearer

---------

Co-authored-by: Jaap de Ruyter van Steveninck <32810691+deruyter92@users.noreply.github.com>