Separate Gtk related code from core logic: info

Surprisingly straight forward, just a couple of places where we really mix
significant logic with UI code (for example setting the window title).

I had to move amount_selected from display-gtk.h to display.h - I guess
the number of dives that are selected is UI independent. But I wonder if
we still will track this as a global variable in a Qt UI (since the Gtk
selection logic is the main reason this existed in the first place).

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

This should make no difference to functionality.

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