Fix for the grid to stop accepting icons if no space left on screen
New feature to save grid status in an external json file

 * the file contains number of icons loaded, displayed, and a flag to say if grid is full.
 * it is created upon icon display completion, both at startup and refresh times.
 * the filename includes the display number to separate local screen from vnc sessions
fixed minor bug when detecting a full grid
Merge pull request #66 from KanoComputing/maxgrid

Fix for the grid to stop accepting icons if no space left on screen