Drop the Workspace::slotSwitchWindowFoo methods

Summary:
They were only delegating into switchWindow(Direction), so let's do the
same through std::bind in kwinbindings and delegate to it directly in
scripting's WorkspaceWrapper.

Test Plan: Not yet, want to add test case for Workspace::switchWindow

Reviewers: #kwin, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6791