Fix launching .desktop files with field codes

.desktop files can add field codes to the EXEC field to specify extra data needed to launch a program. For example, %F will be replaced by a list of files separated by blank spaces, that will be processed as a command line parameter by the program being launched. Unfortunately, the current code in desktop-icons extract that line as-is and uses it to launch the program without doing any replacement, so .desktop files that include field codes fail to launch.

This patch fixes this by searching and removing those field codes when extracting the EXEC string.

Fixes #27 (closed)