Input box completion support

- VSCode Version: 1.17.0 - OS Version: Windows 10 x64 I'm not sure if I'm searching for the right terms, but there doesn't seem to be a way to get an extension to show an input box with completion entries, similar to the 'Show all commands' dropdown, or the 'Go to symbol in workspace' dropdown:  The closest that I can get is to use `showInputBox` with a `validateInput` input validator, but this only allows you to return one entry, which doesn't work well if there's multiple options that could be possible. There's also the quick picker, but as far as I know, it doesn't allow you to dynamically update the available options depending on what has been typed in. Is it possible to expose a similar sort of functionality in the vscode API?