$.ui.suggester: Added isSearching()

Querying isSearching() allows components using the suggester to prevent subsequent actions while
searching is in progress.
I'm not happy with how this variable is used for two very different things. My main problem is: I don't think it can be guaranteed all implementations of `setTimeout` are returning a value of type `number`. Could be a resource type or something. I suggest to set the default to `false` and switch all `typeof this._searching === 'number'` checks to `typeof this._searching !== 'boolean'`.