Dropdown list for egui.

```toml
[dependencies]
egui-dropdown = "0.11"
```
```rust
// Working example can be found in `examples/dropdown.rs`
ui.add(DropDownBox::from_iter(
));
```
Although it's called `DropDownBox`, technically speaking it should be called `ComboBox`.
But this is what egui uses for its version of the widget so yeah.