ComboBox should show full list on initial open

[bmingles]

When using table data sources, ui.combo_box currently only shows filtered items on open if there is a selection. This is different than how React Spectrum ComboBox typically works as well as ui.combo_box configurations that use static item data sources. This has been reported in the plugins repo ComboBox table source behavior inconsistent #1024, but it will likely need to be addressed in the hooks that handle windowed data that live in web-client-ui.