Currently, the "Lower Window" shortcut doesn't have any effect when applying
it to a transient because we lower the transient in the unconstrained stacking
order and then we "raise" it back when building the constrained stacking order.

If we lower a transient, make sure that main clients are lowered too.

• Assigned a shortcut to "Lower Window" in System Settings;
• Opened Konsole;
• Opened Kate and its preferences window;
• Pressed the shortcut.