PopupMenu: Add Item tooltipText

[zsliu98]

See:

https://forum.juce.com/t/how-to-put-a-tooltip-for-a-popupmenu-item/54529

For now, it is impossible to add a tooltip for each item of a PopupMenu without a complex implementation of CustomComponent. Now we can add it by setting tooltipText of an Item. Example:

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);