nd: Merge display_test and display_new logic
nd: Backends may do placement on their own
nd: Better update logic

Now backends may have a proper update action and we change the surface
in-place
Self-ordering backends can then work as expected
linux: Add surface_update function
xcb: Add surface_update function
dbus/helper: Better target and error handling
eventd: Add pause/resume mechanism
nd: Try to attach default targets on start

Add default targets to both xcb and linux backends
dbus: Remove helper

Obsoleted by the new auto-attach behaviour of the nd plugin
eventd/sockets: Verbose existing UNIX socket error
eventd/control: Verbose no control socket error
eventc: s/client type/event category/
eventc: Move event name from options to extra args
eventd/control: Cast GIOStream earlier
eventdctl: More verbose error message
eventdctl: Cast GIOStream earlier
eventdctl: Print user messages without g_log
eventdctl: Rework send command helper

Now takes additional args directly as argc/argv
eventdctl: Do not check plugin commands
eventd/control: Answer eventdctl about the result