add custom gestures by example

[achimmihca]

Would be nice if custom gestures could be added by example.
A similar feature is available in gestop

The idea is as follows:

• user starts custom gesture recording
• user performs gestures
• landmarks of the hand are recorded and written to file
• machine learning model is trained with this data

The process can be repeated multiple times.
Note that it is also necessary to train the gestures that should NOT be recognized.

PyTorch is probably a suited Machine Learning library for this task