gtk4-prep: replace direct GdkEvent struct access with accessor functions

[Arecsu]

Replace all direct GdkEvent struct member accesses:

• event->type
• event->button
• event->x
• event->y
• event->state
• event->keyval
• event->time
• event->window
• event->scroll.direction
• event->delta_x/event->delta_y
• event->direction
• event->hardware_keycode
• etc.

With the corresponding gdk_event_get_*() accessor functions via thin inline wrappers in src/common/gdk_event_utils.h.

The wrapper functions (dt_gdk_event_get_*) accept const void* so any typed event pointer (GdkEventButton*, GdkEventKey*, etc.) can be passed without an explicit cast. They return the field value directly instead of using out-parameters.

This removes ~500 direct struct accesses across 65 files. Remaining direct accesses are limited to:

• Synthetic event construction (writing to gdk_event_new() events)
• Event mutation before forwarding
• Fields without GTK3 accessor functions (configure.width/configure.height, crossing.mode/crossing.detail, key.is_modifier)

See https://docs.gtk.org/gtk4/migrating-3to4.html

Stop using direct access to GdkEvent structs

In GTK 4, event structs are opaque and immutable. Many fields already have accessors in GTK 3, and you should use those to reduce the amount of porting work you have to do at the time of the switch.

Related: "Stop using direct access to GdkEvent structs" #15920 #20433