Feature: rotate rectangle box

[juliusHuelsmann]

For the application I am likely to use the labelImg tool for, rectangular objects need to be classified from above. These objects can be rotated.

Therefore, it is necessary to be able to rotate the currently active shape, e.g. by displaying an additional vertex in case objects are selected, that provides on click drag functionality for rotating the shape.

[vdalv]

Closing because there's already an open issue for this feature request: #20