Add option to skip units with 0TU

[Marshallx]

When clicking the "don't reselect unit" button, the player is generally finalizing their actions. It makes no sense to have to cycle through units who have zero time units and so cannot be given any further orders. This is especially frustrating when a large number of units have panicked.

This change adds an option to automatically skip over units with 0TU (it does not exclude skipped units from reselection through the "select next unit" button though)