Fix split-view shortcuts

The split-view actions define a shortcut using `Konsole::ACCEL`, which
since !609 expands to `Qt::CTRL | Qt::SHIFT` on systems that aren't
macOS.  This breaks the shortcut for the split-view actions however as
`Qt::Key_ParenLeft` and `Qt::Key_ParenRight` do not work with the
`Qt::SHIFT` modifier.

Using `Qt::CTRL` (Cmd on macOS, Ctrl everywhere else) as the base
accelerator for these two actions makes splitting views work again as
expected.