Add SpreadsheetLoader support for sign-magnitude

The spreadsheet loader defaults to 2’s complement when
extracting signed integers. With this commit, the
representation can be specified between parenthesis
in the raw value column, i.e.: int(2c), int(si).
Just ‘int’ will still default to 2’s.

Bug: 2259
Merge pull request #25 from fqqb/bug2259

Add SpreadsheetLoader support for sign-magnitude
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.