Default data provider of Overview View is hard-coded

[bhufmann]

The default data provider ID of the Overview View is hard-coded to the histogram XY chart of the Trace Compass trace server. The default data provider ID should not be hard-coded, but should be configurable using a preference.

Note: The end-user has now way to know what the IDs of the available XY views are. So, the ID is probably not the best thing to have a preference for.