Make the desktop_file_to_binary function less likely to fall over and do something unexpected.

• Uses a shell implementation (!24 (merged)) of which in the desktop_file_to_binary to avoid tripping over unexpected output from command -v
• In addition it also makes the parsing a bit more standards compliant than it previously was.
• Adds a developer script to easier test internal functions in the xdg-utils-common.in file

TODO:

• Remove gawkism near the awk match() statement.

Fixes: #252 (closed)