annotation widget mac specific issues

[melonora]

For the hackathon we will merge the annotation widget as is but there could be some specific mac issues:

• when saving an element, the View widget is not updated. Workaround: from the element widget add the element, and delete it; now the View widget is updated an it works.
• The colors of the annotation widget do not correspond to the colors used for plotting, easy fix.
• The "Annotation table" dropdown combobox does not update the color classes.
• when trying to plot instance_key an error is raised. The user playing around with the annotation widget may encounter this bug because the table created has only a few columns, so it's likely to try to plot instance_id.

[LucaMarconato]

A reminder to myself (or anyone with a Mac) to check if #261 has fixed the bugs reported above.