[Refresh] Add initial refresh button
[Refresh] Add button class for ContextAwareActions
[Refresh] Add refresh button to center pane

Complete nasa/mct#138 by adding a refresh button
to the center pane (in the title bar of the
content area). Triggers the Refresh Now action.
[Refresh] Unit test ContextAwareButton
[Refresh] Add refresh icon
[Refresh] Use component from persistence on refresh
[Refresh] Update refresh action test
Merge pull request #175 from VWoeltjen/mct138

[Refresh] Add refresh button to center pane, nasa/mct#138