crawl: add .desktop file and use high-res app icon

The crawl source includes .desktop files but currently they are not
installed. This change installs them (with the executable path tweaked
for NixOS compatibility).

Also included in this change is an upstream patch to use the included
high-res app icon instead of the default 32x32 icon. The default icon
is very low res and looks out of place beside other app icons.