Show a file selector in OPK links only if Exec requires a file

Previously, if a user installed a new version of an OPK with the same
name as one that had a link configuration file, GMenu2X could request
a file for an application whose new version required no files to
launch.

In practice, this transition would occur only once per OPK application,
when its developer decided to use a custom browser after having used
GMenu2X's file browser. This bug would then show GMenu2X's file browser
to request a file that would not even be passed to the application.

Thanks to Nebuleon for analyzing the problem and the above description.
Properly save the value of Selector Browser after editing a link