config: add <core><primarySelection>

config: add <core><primarySelection>

...to enable/disable primary selection clipboard support. This only works
on launch.

The reason it is useful to be able to disable this is that some clients
(like browsers) support middle-button-click to start scrolling up/down.

With some clients can be disabled via

   gsettings set org.gnome.desktop.interface gtk-enable-primary-paste false

...but for others (like chromium and electron based programs) a compositor
setting is required.

Fixes: #2815