Redirect "ui_accept" input for specific Tree edge cases.

Trees appear to eat "ui_accept" before `gui_input` receives it. This makes certain use cases--triggering buttons in cells--impossible via the keyboard.

Here, `ScreenReader` attempts to intercept recent presses of "ui_accept" and, if a button in a cell should receive focus, redirects the `InputEvent` to the custom handler for `Tree`.

Unfortunately, this loses the ability to click buttons in cells in the editor's node tree for some reason. It restores, or maybe gets working in the first place, clicking on the _Remove_ button in the input mapping screen. Not sure how to restore the former functionality--these trees are complex and not well-documented.

Accessible.gd

@@ -351,6 +351,8 @@ func text_edit_input(event):
 
 var _last_tree_item_tokens
 
+var button_index
+
 
 func _tree_item_render():
 	if not node.has_focus():
@@ -372,7 +374,15 @@ func _tree_item_render():
 					tokens.append("expanded")
 			var button_count = cell.get_button_count(i)
 			if button_count != 0:
+				var column
+				for j in range(node.columns):
+					if cell.is_selected(j):
+						column = j
+						break
+				if column == i:
 					button_index = 0
+				else:
+					button_index = null
 				tokens.append(
 					(
 						str(button_count)
@@ -380,6 +390,7 @@ func _tree_item_render():
 						+ TTS.singular_or_plural(button_count, "button", "buttons")
 					)
 				)
+				if button_index != null:
 					var button_tooltip = cell.get_button_tooltip(i, button_index)
 					if button_tooltip:
 						tokens.append(button_tooltip)
@@ -394,8 +405,6 @@ func _tree_item_render():
 
 var prev_selected_cell
 
-var button_index
-
 
 func _tree_item_or_cell_selected():
 	button_index = null
@@ -430,8 +439,8 @@ func _tree_input(event):
 			node.rect_global_position.y + area.position.y + area.size.y / 2
 		)
 		node.get_tree().root.warp_mouse(position)
-	if item and column != null and button_index != null:
+	if item and column != null and item.get_button_count(column):
-		if event.is_action_pressed("ui_accept"):
+		if Input.is_action_just_pressed("ui_accept"):
 			node.accept_event()
 			return node.emit_signal("button_pressed", item, column, button_index + 1)
 		var new_button_index = button_index

ScreenReader.gd

@@ -173,6 +173,18 @@ var in_focus_mode_handler = false
 func _input(event):
 	if not enabled:
 		return
+	var focus = find_focusable_control(get_tree().root)
+	if focus:
+		focus = focus.get_focus_owner()
+		if focus is Tree and Input.is_action_just_pressed("ui_accept"):
+			var accessible
+			for n in focus.get_children():
+				if n is Accessible:
+					accessible = n
+					break
+			if accessible and accessible.button_index != null:
+				accessible._tree_input(event)
+				get_tree().set_input_as_handled()
 	if enable_focus_mode:
 		if (
 			event is InputEventKey