Fixed issues in kwin_wayland task buttons

[tsujan]

This PR fixes the following problems:

• If a window was minimized by its task button's context menu, it couldn't be restored by that menu;
• If a window and its child dialog were minimized, only its child dialog would be restored;
• The Restore item was always enabled in the context menu; and
• The Maximize item was always disabled in the context menu.

Closes https://github.com/lxqt/lxqt-panel/issues/2093