[Scenario] Skeleton class for activity type control

Add class which will hold implementation of custom
controls for Activity Type.

This supports nasa/MCT-Plugins#103, which requires some
custom UI in the Info View for linking to external
resources on the web.
[Scenario] Add field/icon for custom control

Add a text field and icon to display the URL
for the custom control for Activity Type.
[Scenario] Add URL field to Activity Type model
[Scenario] Add URL editor to Activity Type
[Scenario] Expose custom editor for Activity Type

Expose the custom control for editing and
accessing the URL field of Activity Type
in the Info View.
[Scenario] Use JButton for Activity Type link

Use JButton instead of JLabel for the clickable
icon used to follow links to external resources.
This supports tab focus etc.
[Scenario] Open browser on button click

Follow the provided resource link by opening
a browser, if possible, from the Activity Type's
Info View.
[Scenario] Silently ignore bad URLs

User experience for this situation is TBD
[Scenario] Notify Info View of changes to URL field

When an Activity Type's URL field is edited
(completed by either focus change or enter)
notify the info view, which is connected
via a change listener. This allows the change
to be persisted along with the component.
[Scenario] Show dialog message on bad link
[Scenario] Move strings to bundle for Activity Type

Move strings used in the custom Info View UI for
Activity Type to a bundle.
[Scenario] Add tooltip to URL-opening button

Add a tooltip to the button used to open
linked resources from Activity Type's
Info View.
Merge pull request #106 from VWoeltjen/mctPlugins103

[Scenario] Add URL field to Activity Type