seat: select drag-and-drop action from keyboard modifiers

[coffeejunk]

The drag_handle_keyboard_modifiers implementation in wlroots is empty, so ctrl/shift have no effect on a drag.

static void drag_handle_keyboard_modifiers(struct wlr_seat_keyboard_grab *grab,
		const struct wlr_keyboard_modifiers *modifiers) {
	//struct wlr_keyboard *keyboard = grab->seat->keyboard_state.keyboard;
	// TODO change the dnd action based on what modifier is pressed on the
	// keyboard
}

Separately, it was stated that this should be implemented by the compositor swaywm/sway#8219 (comment)

Map the modifiers to wlr_data_source::compositor_action on each modifier's event while a drag is active: ctrl copies, shift moves, ctrl+shift asks.

As an example, Thunar:

Without the patch, dragging a file between two windows while pressing CTRL moves it:

no-dnd-modifier.mp4

With the patch applied, dragging a file between two windows while pressing CTRL copies it:

dnd-modifier.mp4

[ammen99]

If we store the active_drag object, then we don't need drag_active at all. The latter should be removed, and all the places that use it can simply compare the active drag against a null pointer (or, if you prefer, you could add a helper function is_drag_active() which does the check). Otherwise lgtm.