KFileItemActions: Add logic to order actions between separators

Commit 33785332 has purposedly ordered
actions by their name. However, this breaks separators in service context
menus.

For example:

Actions=progressive;optimize;\_SEPARATOR\_;compress_50;compress_70;compress_75

The separator would end up at the end of the list:

Actions=compress_50;compress_70;compress_75;progressive|optimize;\_SEPARATOR\_

And since the logic check if the separator is the last item to add it:

!actions.last()->isSeparator()

separators basically disappear from service context menus.

The purpose of this commit is then to create groups of actions between
each separator. Actions will be sorted within these groups, but it will
keep service menus original order.

BUG: 466115