taskswitcher: reintroduce maximizing task on activating it in switcher

in my task switcher rewrite to add the fancier gestures I've removed that activating an app (= selecting it in the switcher) maximizes it so use cases with floating windows in docked mode don't break/get annoying. I thought this was fine since we configure KWin to maximize windows when not in docked mode anyways but there are cases where this can break and then there's no way for users to re-maximize windows anymore.

This MR re-enables that but checks whether the shell is in docked mode and only does that when not in docked mode. It's technically a hack/workaround, but I don't have a better solution yet.

Fixes https://bugs.kde.org/show_bug.cgi?id=492077

IMO could/should be cherry-picked for 6.2