Dropdown<T>.GenerateItemText flow needs reassessment

[frenzibyte]

Recently, a series of pull requests got opened and merged back and fourth to resolve edge cases all revolving around GenerateItemText:

• Fix Dropdown calling GenerateItemText before it's loaded #5516
• Revert Schedule changes to Dropdown #5794
• Fix automatic renderer dropdown item not showing actual renderer name osu#23858
• Remove Dropdown.AddDropdownItem(text, value) overload #5831
• Fix Dropdown calling GenerateItemText() before it was fully loaded #5836
• Fix item labels not generated while dropdown is in Ready state #5850

While GenerateItemText might behave more properly right now, I believe going forward, the best place I can imagine for calling GenerateItemText would be somewhere within the DrawableMenuItem itself when the item label is actually needed for display.

That way would guarantee both the Dropdown's BDL to be called first, and guarantee the label assignment happening while the drawable menu item is loading (rather than after it's loaded, potentially causing a performance hit or wrong one-frame display). All without going through checking load states or scheduling anything.