SizeAwareStack

This is a simple widget stack selecting the best fitting widget. 
The algorithm is very basic and requires children sorted by size, smallest first.

It fixes bug https://bugs.launchpad.net/mixxx/+bug/1374214
```
										<SizeAwareStack>
										    <Children>
						                       <WidgetGroup>
						                       		<MinimumSize>10,10</MinimumSize>
						                       		<MaximumSize>10,10</MaximumSize>
						                       		<Style> QGroupBox {
														background: blue;
													}
						                       	    </Style>
						                       </WidgetGroup>
						                       <WidgetGroup>
						                       	    <MinimumSize>100,10</MinimumSize>
						                       	    <MaximumSize>100,10</MaximumSize>
						                       	    <Style> QGroupBox {
														background: red;
													}
						                       	    </Style>
						                       </WidgetGroup>
						                       <WidgetGroup>
						                       		<MinimumSize>200,10</MinimumSize>
						                       		<MaximumSize>200,10</MaximumSize>
						                       		<Style> QGroupBox {
														background: green;
													}
						                       	    </Style>
						                       </WidgetGroup>
						                    </Children>
						                </SizeAwareStack>
```
It can be tested with shade_dev from developer_skins
If I understand the underlying Qt code right, it should not resize because of the new widget, because all sizes are calculated in the layout from all stacked widgets. 
I also have not seen one during my tests. 
But even if there is an other resize event, the new widget should fit to the new size. so there is no second change in that theoretical case.