Enhance PathUtilities.SelectFileInExplorer method

Some file managers on Linux can select a file. This change implements
this for nautilus and nemo. Also add new button to test app to
run this method.

Change-Id: I76160389c45bb9085a5eca3e4adbb4b4a33cd383
Although this is currrently a private method,  I could imagine us one day making it public. I wonder whether it would be better to make this method return xdg-open rather than null when something goes wrong on Linux?