Visualise pose tracks as a napari Points layer

[niksirbi]

A core feature of the envisioned napari frontend (see #31) is the ability load pose tracks from DeepLabCut and SLEAP.
We need to write readers that wrap around the functions in load_poses.py.

[niksirbi]

We decided to not implement this feature as a reader plugin. The main reason is that we need to also pass the source software when loading a file, the file path alone is not enough.

Instead we are making a loader widget as part of the movement napari plugin. Users will be able to specify file path, fps and source software through the widget. See ongoing work in #112