Q:Let's say I use Witchbane orb to give myself hexproof. Do Planeswalkers I control have hexproof as well?
No. A Planeswalker can still be targeted as a permanent. Example, it can still be affected by Beast Within. Having Hexproof just prevents from targeted burn spells from targeting the player and then being redirected to the Planeswalker.
306.7. If noncombat damage would be dealt to a player by a source controlled by an opponent, that opponent may have that source deal that damage to a planeswalker the first player controls instead. This is a redirection effect (see rule 614.9) and is subject to the normal rules for ordering replacement effects (see rule 616). The opponent chooses whether to redirect the damage as the redirection effect is applied.