reduce flickering by 'debouncing' user input

[Thibaut Brandscheid]

Problem
-----------
Every time the user changes the content of the search-bar the Dash updates its result-view. When typing, this results in result-view screen flickering.

Common solution for this problem
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Is to cache the last user-input and search only after a delay, as long as there has been no new user input. This function is called 'Debounce' in ReactiveX -> http://reactivex.io/documentation/operators/debounce.html

Pros:
* less screen flickering
* less resource usage (the search-query is often executed only once)

Cons:
* very slow search results take from a user point-of-view longer, because they get executed after the debounce-delay.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: dash 0.5.8-2.1ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-11.26-generic 4.4.4
Uname: Linux 4.4.0-11-generic x86_64
ApportVersion: 2.20-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Mar 10 17:18:23 2016
InstallationDate: Installed on 2015-12-12 (89 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Alpha amd64 (20151209)
SourcePackage: dash
UpgradeStatus: No upgrade log present (probably fresh install)