Dropdown list for egui.

toml
[dependencies]
egui-dropdown = "0.1"
``rust
// Working example can be found inexamples/dropdown.rs`
ui.add(DropDownBox::fromiter( &self.items, "testdropbox", &mut self.buf, |ui, text| ui.selectable_label(false, text) )); ```
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.