Better default Linux sketchbook location

The Linux version now doesn't prompt for a sketchbook location the
first time you start the program.  Instead it now, like the other
platforms, creates a "UECIDE" folder in your documents folder
(/home/foo/Documents/UECIDE) for your sketches.  Of course, if you
already have a sketchbook folder elsewhere set up in the preferences.txt
file, it will keep using that.