User-defined shapes

**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