Revamp data selection applet GUI.

These patches remove the "Add" and "Remove" buttons under the tab view of the
data selection GUI. The "Add" button is moved to the last row of the raw data
tables, similar to the placement used in the summary view. Remove button is now
shown as an overlay on the active row of the raw data table.

The overlay is admittedly a bit hacky. It works on my highly customized linux
desktop. Please test on different platforms and give feedback here.