David Fellows - 2018-03-12

The attached patch Implements my suggestion. It adds a new option, XTRKCAD_OPENDESKTOP_COMPLIANCE, in the top level config file
which controls where xtrkcad.desktop and xtrkcad.png are installed. It defaults to ON which is current behaviour. I f OFF they are installed in install_dir/ share/xtrkcad along with everything else. Of course if they are put there, they will not be found automatically by desktop manager configurators, but for testing I run xtrkcad from the command line.