debugui: add Dropdown

[Vmarcelo49]

Implemented a dropdown menu for debugui.

It allows listing selectable items, but there's still an issue: widgets behind the dropdown remain interactable, even when the dropdown is open and overlapping them.

Usage is just like the other widgets

ctx.Dropdown(&pointerToIndex, sliceOfStrings).On(func() {
					doSomething())
				})

[hajimehoshi]

Let's add a dropdown to example/gallery

[Vmarcelo49]

I have made the changes, removed the Dropdown type, renamed that variable and put one example of the dropdown into the gallery, into a window

[hajimehoshi]

The code seems generally good.

[Vmarcelo49]

I corrected wrong licence
Removed some obvious comments, keeping only the specific ones about the dropdowns
removed isOpen variable that was used only once

Now the code seems pretty clean

[hajimehoshi]

(Sorry I've not approved yet)

[hajimehoshi]

LGTM, thanks!