Allow extending EventViewer with extra columns

The current implementation needs to be configured
in the event-viewer.yaml file.
EventViewer: Make colors of warnings/errors easier on the eyes
Prevent NPE when no extra evt cols defined
Save column order of EventViewer in UI prefs

Save the column of an order with respect to the other columns in
the user preferences when the user closes the window.

When something in the configuration has changed (e.g. extra columns
are no longer present), it will do its best to restore the default
Event Viewer look.

This commit also brings a couple of changes to borders and colors
that appeared while testing the Event Viewer on Mac.
Merge pull request #27 from fqqb/evt-viewer

Extra columns in EventViewer