[IZPACK-1114] Next panel not gathered correctly if it depends on panel conditions

There is an issue when changing panels using panel conditions:
If a panel depends on conditions based on variables set by a previous panel (like $INSTALL_PATH in TargetPanel), these conditions are not evaluated correctly, because at the evaluation time there are still not updated the variables set in the previous panel. This results in an unexpected panel order although the conditions are shown correctly in trace mode after the panel change (when showing them in trace mode the variable update has been already passed, in difference to the moment when gathering the next panel based on conditions).

There have been also made some small optimizations to avoid repeated evaluating on the same panel under the same circumstances.