Bug#956760: cairo-dock-plug-ins: (Build-)Depends on deprecated libindicator

[Kentaro Hayashi]

FYI: 

cairo-dock-plug-ins was already migrated to libayatana-ido3-dev,
but still use libindicator3-dev [1] instead of libayatana-indicator3-dev.

[1] 
https://salsa.debian.org/cairo-dock-team/cairo-dock-plug-ins/-/blob/master/debian/control#L27