Onboard interferes with Cinnamon's built-in keyboard

Hi,

Cinnamon ships with its own onscreen keyboard built on libcaribou. When onboard is installed, both keyboards popup to respond to AT-SPI events.

To prevent this from happening:

Please make the following changes to stop Onboard from autostarting in that environment:

- Add Cinnamon and X-Cinnamon to the list of only-show-in in the XDG autostart Exec.
- Add Cinnamon and X-Cinnamon to the list of NotShowIn in the application desktop file (for the main
menu)

data/onboard-autostart.desktop:
Exec=onboard --not-show-in=GNOME,GNOME-Classic:GNOME,Cinnamon,X-Cinnamon

data/onboard.desktop.in:
NotShowIn=X-Cinnamon;

Cinnamon 3.4 worked around this issue by registering Onboard's DBUS name, thus preventing it to work. This wasn't ideal at all.

This workaround will be removed in Cinnamon 3.6. We'll rely on Onboard not autostarting in the Cinnamon DE.