Now what if I want to start a chat quickly without opening the contact list, you might have asked yourself? Well, you are not out of luck. What you need to do is install an additional plasmoid that contains a few detachable widgets. It is called Lancelot and can be obtained by installing kdeplasma-addons.
Unlock the widgets if necessary by right clicking on the desktop, then right click again and choose "add widgets" (I'm using German KDE, so it might read differently in the English localization). Select the Lancelot launcher (not the component) and drag it to some free space on your desktop or the panel.
That's a nice menu, isn't it? The best thing about it is that you can drag the title of a section and tear off a copy, which can then be placed anywhere as an independent plasmoid. Tear off the contacts section and place it on the desktop or in the panel, and you have a nice widget that displays your online contacts and allows for starting a conversation with a click, or by just hovering the mouse over the right hand arrow (that no-click behavior can also be switched off if it sucks for you). You can remove the Lancelot launcher afterwards if you don't need it anymore.