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
Requiring an ordering feels awkward to require the author to do. Why not just search for the smallest one?