[Scenario] Rename visual control for links

Rename ActivityTypeVisualControl to
LinkVisualControl, in preparation for
reuse of class as a part of the visual
control for Activity components.
(Supports nasa/MCT-Plugins#103)
[Scenario] Add link control to Activity info view
[Scenario] Support URL in Activity custom control

Adds wrapper class used to communicate URL
(in addition to existing tag-like categories)
between Activity Component and its custom
control in the Info View.
[Scenario] Add URL field to Activity model

Adds field to store external link in
Activity model and connects this to
user interface in the Info View.

This completes amendment of solution
to nasa/MCT-Plugins#103 to include
links on Activity objects (not just
Activity Type objects)
Merge pull request #117 from VWoeltjen/mctPlugins103b

[Scenario] Add URL field to Activity