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: Dirk Hohndel <dirk@hohndel.org>
Fix some of the gcc-4.8 warnings

Most of the warnings are IMHO false positives:
e.g.: an enum variable is initialized in a switch statement that has a case for
      each possible enum value - yet gcc 4.8 warns that it could be used
      uninitialized;
or:   two variables are initialized together in the code - second one of them
      is previously initialized to -1 at declaration time, both are initialized
      in an if (second one == -1) clause - so they are guaranteed to both be
      initialized...
I did not "fix" those as the code is actually correct.

But there are three spots where it catches things that could indeed go wrong
(with odd input data in one of them).

This commit also adds a check to only call g_type_init() for older versions of
glib as in newer ones it is deprecated.

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