Converted the Buttons to JFace Actions. This has two advantages: No
handling of SWT components (no SelectionListeners), and easier to add to
different SWT objects (ToolBars and Menus). These Actions are also
configured to be enabled only when the VisItSwtConnection is not null.

I also simplified the constructors and added a little documentation.
Merge pull request #8 from jdeyton/time_varying_widget

Converted the Buttons to JFace Actions.