Fix Connection dock's popups always allowing disconnect

[Mickeon]

Fixes the issue described in this comment as a regression from #81221.

A Connection Dock's TreeItem needs to be selected for the connection's PopupMenu, called slot_menu to update accordingly on the spot. However, this doesn't happen. It means that it is either possible to disconnect inherited signals, or the opposite, at seemingly completely random. This PR fixes that.

[dalexeev]

This fix looks necessary for the TREE_ITEM_TYPE_SIGNAL case as well.

[Mickeon]

That is correct. The issue was just less apparent because most item options worked regardless. I pushed a fix for the signal item type as well.

[dalexeev]

Maybe do this above, for future proofing? And you can also add a comment.

	TreeItem *item = tree->get_item_at_position(mb_event->get_position());
	if (!item) {
		return;
	}
+	
+	if (item->is_selectable(0)) {
+		tree->set_selected(item);
+	}

[Mickeon]

Sure thing. I tested it again and it worked identically. A comment, too.

[dalexeev]

I haven't tested this, but it makes sense to me and I trust @Mickeon. Thank you!

[KoBeWi]

tbh this code should be reworked to use item_mouse_selected instead of gui_input, but for now this solution works.

[dalexeev]

tbh this code should be reworked to use item_mouse_selected instead of gui_input

This was changed in #81221 because class items are not selectable.

[akien-mga]

Thanks!