wacom: Don't leave stray timelines on the calibration UI

Keep the reference for the error/helper messages animations in
the CalibArea struct, so those are destroyed and not leaked, or
possibly crashing when running on already destroyed actors if
the dialog gets cancelled at the right time.

https://bugzilla.gnome.org/show_bug.cgi?id=719698
wacom: Protect calibrator UI to spawn animations multiple times

The window state may be updated more than once, so only initiate
the UI and animations when it's first called.

https://bugzilla.gnome.org/show_bug.cgi?id=719698
wacom: Fix first animation of calibration UI

The "target" was seen moving from 0,0 to the first calibration
point, so 1) avoid target relayouts when the window is still
being positioned and 2) start the "target" actor as hidden so
it isn't seen moving from anywhere when first shown.

https://bugzilla.gnome.org/show_bug.cgi?id=719698