[TabBox] Remember current model index on SwitcherItem model set

In case we need to set the SwitcherItem model in
TabBoxHandlerPrivate::show(), remember the current index row,
otherwise it gets reset to the first item.

Reviewers: KWin, graesslin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D4162

BUG: 333511