wibox.layout: Add fixed and flex layouts widget spacing property

**Why: **
Because it look nice and is better than adding 10000 million margin widgets. Other toolkit also have such option

**Testing done:**
* It does not crash with giant red popup
* Flex fit is the same with and without spacing
* The align is fit_0_spacing+(#widget-1)*spacing

**Screenshot:**
![blender219](https://cloud.githubusercontent.com/assets/340384/4688646/846db996-5686-11e4-9bde-9b257f6a5bae.png)
(see the tag and tasklist)