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
Should probably use SpecialFolder.LocalApplicationData to get $HOME/.local/share, and SpecialFolder.CommonApplicationData to get /usr/share