Configurable layer speeds per object

Hey guys! I wanted to have a possibility of slowing down (or speeding up) the print for the whole layer within one of the objects on my build plate. It came to my mind today (after five failed prints) that it may be easily doable in the _Object Settings / Layers_ configuration dialog, so I simply added a column to the dialog.

![slic3r speed screenshot](https://cloud.githubusercontent.com/assets/3009963/2810217/c51e64ca-cda8-11e3-9951-6f8689f80fdf.png)

The Layer object needed to be extended by adding the speed attribute to it. And this attribute is then used to alter the speed in the GCode. Simply by dividing the standard speed by this user defined percentage.

Please note, that I've never done anything in Perl and I'm also fabulously lazy to read any manuals, coding standards and such a kind of stuff. Please kindly tell me if anything is wrong with this little piece of change and I may try to do it better next time. Otherwise, I'm glad enough if everyone enjoys this new feature. Even if you perhaps decide to choose another implementation scenario.