samigo: Hot Spot questions inaccessible to keyboard or screen reader users

The Hot Spot/image map question type is inaccessible to screen reader users (even sighted ones - it is not possible to add any text alternative to any portion of the image).  It is also impossible for keyboard-only users to navigate and answer.

Ideally, the following fixes should be made:

• Allow (possibly require) instructor to add alternative text for each region in the image map.
• Allow students to navigate and select answers with a keyboard.

If the above are not possible to add, the following warnings are needed for users:

• Warning to instructors on the authoring screen that alternative questions may need to be created for students who use a keyboard or screen reader.  (It may be helpful to add such a message even if the above fixes can be made).
• Warning to students that this question type must be answered with a mouse/pointer device and screen reader users should skip it.