Variable substitution in Info.plist files broken for '@VAR@' syntax

I was using the @ICON@ style for variables in my Info.plist file and since Qbs 1.14, those replacements no longer work.

This turned out to be due to change ad11bb7383bf730ca0241af24a8b4179af157c7f, since now it searches for the last occurrence of "@" and then tries to find the next, which never works.

I realize this notation is not actually documented (at least not mentioned at https://doc.qt.io/qbs/qml-qbsmodules-bundle.html#processInfoPlist-prop) so it might be fine to remove it entirely rather than fixing it up. A potential fix would be to add a flag to set the search direction for each of the possible syntaxes, or to find the first index when "syntaxes[i].open == syntaxes[i].close".