Allow inline svg handling Variable elements in skins

This is the same pr as https://github.com/mixxxdj/mixxx/pull/285 but cleaner :)

I needed the skin parser to be able to manage Variable elements in svg for cues and loops buttons so I added a function to do it and hooked it directly if a svg element is present in a (un)pressed element.

So now, I can handle inlined svg in my pushbuttons like this : 
```xml
<Pressed>
	<svg height="16" width="16">
		<rect x="0" y="0" width="16" height="16" style="fill:#000000; stroke-width:1; stroke:#ff0000; stroke-linecap:sqare;" />
		<text x="0" y="15" style="fill:#ff0000; font-size: 16px; font-weight: bold;">
			<Variable name="number"/>
		</text>
	</svg>
</Pressed>
```
As asked, here is the cleaned branch (thanks for the git tricks).

So now some questions/todo list :
+ I used QDir in getSkinPath to be able to use my QTemporaryFile. Latelly, I realized I had to place it in /tmp because I couldn't use the autoremove (the file is on close but I had to close it before setPixmap...). So it's not required for this feature but it looks more "Qt friendly" so I let it.
+ I'm not sure that creating a temp file is the best way.
+ I'm not sure there is a way to set attributes depending on some vars (css would be the good practice normally but are they supported now?).
+ Variables are not handled in svg file, only in inline svg.
+ Only WPushButton supports inline svg for the moment.


I think that the best approach is to remove the use of a temp file by making the Paintable class able to use a QByteArray as input but without certitude. 

You're insight is welcome!
please use `QDir::tempPath()`. WIndows does not have a `/tmp/` folder