FR: add tooltip for each item of PopupMenu

See:

For now, it is impossible to add a tooltip for each item of a PopupMenu without a complex implementation of CustomComponent (I give it up). However, we can achieve it by adding several lines of code. And the usage is very simple:

const auto menu = comboBox.getRootMenu();
juce::PopupMenu::Item item;
item.itemID = 1;
item.text = "text";
item.tooltipText = "tooltip text";
item.isEnabled = true;
item.isTicked = false;
menu->addItem(item);