[Feature Request]: Searchable dropdown boxes in Hop (especially for variables)

### What would you like to happen?

It would be wonderful if the dropdown boxes in Hop were searchable.

The example use case is a long list of environmental variables in Hop where you want to search typing out a word in the variable you want to use. For example: "Pr" might jump to the PR section where you see "PROJECT_HOME" among other things. The list might even filter down to variables that have the block of characters in them.

That would boost usability a lot.

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui

The issue identifies the Hop Gui component but names no files, tests, or entry points. Start by locating the dropdown implementation used for environmental variables, then determine how a typed string should search or filter its options; done means long variable lists can be searched and the requested option is easy to select.

Written by the indexing model from the issue text.