Accessing GdkDisplay before Gdk was initialized is not allowed.
Gtk now ensures the init was performed (see gtk!7836), which makes
GHex crash on starting, because `ghex_init_configuration()` tries
to create a GtkCssProvider, which depends on displays.

Rework the GHex initialization to avoid the problem:

- add an application `::startup` handler, executed once after Gtk/Gdk init
- move `ghex_init_configuration()` to `startup()`
- also move the setting of "gtk-dialogs-use-header" there,
  no need to do it at every `::activate`
- remove redundant `gtk_window_set_application()`, already done by `do_app_window()`
- remove redundant `g_application_register()`, the GApplication takes care of it

(cherry picked from commit 2e1946f6)