Fix clicking on padded dropdown search bars not opening the menu

[smoogipoo]

Fixes ppy/osu#28594

In hindsight, it's probably not a good idea to handle click-to-open input in something like a "search bar".

In osu!, the search bar is padded to add space for the down-arrow icon: https://github.com/ppy/osu/blob/9039ec34bae03a9b20e43b2161cc5c9e767c3889/osu.Game/Graphics/UserInterface/OsuDropdown.cs#L405-L408

I also explored the option of moving the padding to the textbox proper, but textboxes don't seem to properly handle padding at all.

[smoogipoo]

I was just about to remove this, and ended up adding it back. I want to explain this a little bit for future reference.

"Why isn't this just return true;?"

Originally, this condition was required because of an interaction with the code changing the menu state when the textbox is focused/unfocused (keep in mind the focus of this entire component is handled by the textbox alone):

osu-framework/osu.Framework/Graphics/UserInterface/DropdownSearchBar.cs

Lines 114 to 123 in 94cd76a

	/// <summary>
	/// Opens or closes the menu depending on whether the textbox is focused.
	/// </summary>
	private void updateMenuState()
	{
	if (textBox.HasFocus)
	dropdown.OpenMenu();
	else
	dropdown.CloseMenu();
	}

Basically, we need to make sure the textbox receives focus at some point, hence input needs to pass through when the dropdown is supposed to be visible, and not pass through when it's supposed to be invisible.

By sheer luck, and as a result of a separate bugfix, this is now also handled by:

osu-framework/osu.Framework/Graphics/UserInterface/DropdownSearchBar.cs

Lines 158 to 163 in 94cd76a

	else
	{
	// This exists because the menu is _sometimes_ opened via external means rather than a direct click.
	// _Sometimes_, this occurs via a click on an external button (such as a test scene step button), and so it needs to be scheduled for the next frame.
	Schedule(() => dropdown.ChangeFocus(textBox));
	}

That specifically handles the case where the dropdown is opened externally, i.e. by calling Dropdown.Menu.Open().

Given that the above is a very roundabout way of handling this, I prefer to be explicit about intentions here.