Take 1..n KMainWindows in kRestoreMainWindows

Summary: We've got C++11 and can turn KMainWindow::kRestoreMainWindows into a variadic template to support more than 3 different toplevel widgets. Gives also minor code deduplication.

Test Plan: Compile and execute kmainwindowrestoretest.

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16494