Add functions gpa_show_info and gpa_show_warning.

* src/gtktools.c (gpa_show_info): New.
(gpa_show_warning): New.
(gpa_window_message, gpa_window_error): Make them wrappers fro the new
functions.

Using the message dialog functions is better than doing it ourselves.
Note that a title bar is not anymore show but that is compensate by
the fact that these dialogs do not anymore show up in the taskbar.