We should not have essential signal connections (such as setting tool
options from brush properties) implemented in the tool options GUI
files, because they are not active until the options GUI is created.
Also, that magic is simply too hidden in the options GUI files.

Move the signal connections and the brush property copying code to
gimppaintoptions.c where is can also be done cleaner.

However, this must only be done for the main tool options instance
that is used for the GUI. Therefore, add a "gui_mode" boolean to
GimpToolOptions and set it to TRUE for all main tool options.

(this is ugly, but much less ugly and much less hidden than all the
places where code lives (like tool_manager.c) that can now be moved
into GimpToolOptions and its subclasses, and implemented cleanly
there).

(cherry picked from commit cb0e6c65)