desktopToDarwinBundle: Implement %F and %U Exec field codes

Similar to the implementation of the `%f` and `%u` field codes. In this
case the amount of arguments passed poses no problem but the position
could, at least in theory.

This finishes the implementation of all the non-deprecated field codes.
As a part of that, repetitions of field codes are left alone. Originally
all field codes were removed. Now we replace only the first occurence.
This is correct for at least `%f`, `%u`, `%F` and `%U` because at most
one of them is permitted.

Shortcomings:

  1. We replace `%[cfFikuU]` patterns one at a time. This means if the
     right field code appears as part of the rest of the `Exec` field or
     in a field code that was substituted earlier.

  2. If any field code is repeated, only the first occurence is
     substituted.

pkgs/build-support/setup-hooks/desktop-to-darwin-bundle.sh

@@ -179,7 +179,7 @@ processExecFieldCodes() {
   local -r icon=$(getDesktopParam "${file}" "Icon")
   local -r execNoKCI="${execNoKC/\%i/${icon:+--icon }${icon}}"
   local -r execNoKCIfu="${execNoKCI/\%[fu]/\$1}"
-  local -r exec="${execNoKCIfu// %[FU]}"
+  local -r exec="${execNoKCIfu/\%[FU]/\$@}"
   if [[ "$exec" != "$execRaw" ]]; then
     echo 1>&2 "desktopToDarwinBundle: Application bundles do not understand desktop entry field codes. Changed '$execRaw' to '$exec'."
   fi