gtk4-prep: stop using GtkButton image API (#15920)

[Arecsu]

Replace:

• gtk_button_set_image()
• gtk_button_set_always_show_image()
• gtk_button_set_image_position()
• gtk_button_get_image()
• gtk_button_get_image_position()

with manual GtkBox packing for the Lua button widget. These are all removed in GTK4.

Guard against gtk_button_set_label() replacing our box (GTK3 creates a new label child and destroys the custom child), show newly replaced images, and force relayout after orientation changes.

Also refactored the code to clean things up.

I used this lua script to test it, no errors. Even solved some pre existent assertion errors:

local dt = require "darktable"
local logo = "./build/share/darktable/pixmaps/dt_logo_128x128.png"
local cb = function() print("ok") end
local b1 = dt.new_widget("button"){label="a", image=logo}
b1.image = logo  -- set same image again
local b2 = dt.new_widget("button"){label="b"}
b2.image = logo
b2.label = "b2"  -- set label after image
b2.image_position = "right"
local b3 = dt.new_widget("button"){image=logo, label="c"}
b3.image_position = "left"
b3.image_position = "top"  -- change position twice
local b4 = dt.new_widget("button"){label="", image=logo}
dt.register_lib("test_final","test final",true,false,{[dt.gui.views.lighttable]={"DT_UI_CONTAINER_PANEL_RIGHT_CENTER",5}},
  dt.new_widget("box"){orientation="vertical",
    dt.new_widget("button"){label="no image", clicked_callback=cb},
    dt.new_widget("button"){label="left", image=logo, image_position="left", clicked_callback=cb},
    dt.new_widget("button"){label="right", image=logo, image_position="right", clicked_callback=cb},
    dt.new_widget("button"){label="top", image=logo, image_position="top", clicked_callback=cb},
    dt.new_widget("button"){label="bottom", image=logo, image_position="bottom", clicked_callback=cb},
    dt.new_widget("button"){image=logo, clicked_callback=cb},
    b1, b2, b3, b4
  }
)

Related: #15920 #20433

[wpferguson]

@Arecsu should the actual widget code such as dt_lua_button_get_widget_in_box, be in gui/dtgtk.c wthout the lua in the function name, and then just called from lua/wiodget/button.c?