User-defined shapes

[amrlafi]

Is your feature request related to a problem? Please describe.
In many image labelling scenarios a user will need to label a uniform shape. Rather than using the time-consuming polygon everytime, the use should be able to define a shape and re-use it across projects.

Describe the solution you'd like
The user should be able to use UI to create the shape. It will be helpful to use a test image to label, and then create a template of the shape.

The shape can be a mix of other shapes such as rectangle, eclipse, and polygon. For simplicity a polygon can be the default way of creating user-defined shapes.
A new tag should be added, for example . The name attribute should be used to lookup the database for the definition of the shape, or otherwise an id attribute should be considered.

Describe alternatives you've considered
As of now, user can manually use polygons, but really time-consuming.

Additional context
N/A

[timiil]

VERY useful this feature!