Add line to align some element relative to other

Add the possibility to align object relative to other  with line help.
It's my last contribution with new feature, in future i debug musescore to help developpement.

![align](https://f.cloud.github.com/assets/1835871/2477836/71330590-b076-11e3-8e40-e25127bf424b.gif)
![stem](https://f.cloud.github.com/assets/1835871/2477835/652d10ce-b076-11e3-974b-bfd767aa630d.gif)

than'k

Robert