Glassestools

add `get_source_directory()` function to camera_recording.Recording

recording.Recording:
- properly initialize `start_time` field
- add `get_source_directory()` function, that correctly handles relative paths

- signal_sync:
- simplified handling race condition
- can now align by dragging gaze signal instead of point on middle of plot
- can now align gaze signal using arrow keys
- added dependency to ensure ffmpeg binaries are always available on all supported platforms
- added method for getting video timestamps using ffprobe
- no longer transcoding avi and mkv scene videos to mp4 during import, no longer needed with new timestamp getting method
- add class to export a scene video with overlaid gaze marker
- added `fixation_classification.from_plane_gaze()` for running fixation classification on data projected to a plane using [I2MC](https://link.springer.com/article/10.3758/s13428-016-0822-1)
- moved functionality from glassesValidator into `glassesTools.validation` so it can be more easily reused. As part of this move, also reworked the logic and added some small bug fixes
- small crashing fixes

- Added support for importing Viewpointsystem VPS 19
- fix further regression with importing any eye tracker that is not a generic eye tracker
- fix further regression due to update to imgui_bundle 1.6.2
- add code to allow avoiding race condition with signal_gui

- added support for a generic eye tracker type, allowing users to import data from any eye tracker into tools using glassesTools as long as they handle conversion to the glassesTools data format themselves. For generic eye trackers, can set the name to display in the recording info.
- better eye tracker colors and contrast with text in the eye tracker widget

- Bumped imgui_bundle to 1.6.2, made changes to code where needed
- Bumped opencv to 4.11
- minimal Python version is now 3.11, due to imgui_bundle dependency