Separate Gtk related code from core logic: planner

Relatively straight forward, just a handful of places where we call
show_error() (a UI function) from the logic code. In the process I noticed
a few places where error returns weren't dealt with correctly.

Added a new planner.h files for the necessary declarations.

This should make no difference to functionality.

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Forgot to add planner-gtk.c

Oops

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>