[Request] Filter values of Dropdown

[RunninglVlan]

It'd be really cool to have filtering in the Dropdown, something like we see in Add Component dialog. One option would be to use Unity's AdvancedDropdown BTW, I already found some implementation in one of this repo's forks Another example is in BennyKok's unity-toolbar-buttons - commit

[RunninglVlan]

@dbrizov, do you plan to merge @krisrok's pull request (#181)?