KDE: disable super_L (windows key) key shortcut as menu launcher

In KDE Plasma 5.8, the super/win key is also used to trigger the launcher, when pressed and released without combining it with another key. To disable the feature,

1. put this into ~/.config/kwinrc:

   [ModifierOnlyShortcuts]
   Meta=
   

2. then reload the window manager (kwin), either by logging out and logging back in, or with with this command:

   qdbus org.kde.KWin /KWin reconfigure
   

Sources:

• Initial announcement of the feature https://blog.martin-graesslin.com/blog/2016/08/modifier-only-shortcuts-available-in-plasma-5-8/
• An answer from Reddit https://www.reddit.com/r/kde/comments/55udc1/how_do_i_stop_the_meta_key_from_opening_the/d8epzzj/
• Plasma tips on KDE's own wiki, with further tips for Latte Dock users https://userbase.kde.org/Plasma/Tips#Windows.2FMeta_Key