Add type-to-select search feature to user list

[MagnusOxlund]

Sometimes you need to check if a certain user is online. Checking the user list could be optimized with a search feature. An unobtrusive version is the type-to-select feature known from file explorers.

Currently, anything you type while the user list has focus is entered into the text field. This is also the case for the server/channel list.