Conditional Layouts

So i started working on a conditional layout for the desktop. The mobile version should look pretty much the same but as soon as the app has a certain size the layout will change.

There are still a few hacks as you can currently not nest the "Layouts" elements without problems. Also you cannot put Tabs/Pages inside a ConditionalLayout. Also filed a bug for this too.
I noticed that often the pixel size is used and started replacing some by "units.gu(x)".

Finally, I made some changes to QtSpotify. I found that there was an uninitialized pointer that was causing problem on my machine. Also I started using "nullptr" instead of 0 and moved some stuff to a separate file (the QtSpotify classes are generally very huge imo).