Selections plugin: add action link to select all values

Currently it is possible to deselect all previously selected values in the value list. With current improvement it possible to select all values in the same manner. This should be directly seen in the selections demo project.

Due to time shortage I have created a patch from selections trunk in SVN. With feature branch and git, it would be a bit longer. Please discuss at the triage meeting, if this is a valid improvement and evtl. include it in the product after testing.