Minor cosmetic changes to the Release UI

* Make the red/green colors less "aggressive"
* Use images for the legend instead of a text field
* Fix padding/margins/alignments in various places
* Use native splitter (sash) widths
* Make the columns in the project list fit things better
* Make the minimum width of the dialog 640x480 instead of 800x500
* Center the dialog over the parent window
* Give the dialog a title on platforms where it is supported
* Give the dialog the lorry icon on platforms where it is supported

Before:
![image](https://cloud.githubusercontent.com/assets/114601/4080712/2cad31d6-2ee1-11e4-874c-e829f596e90d.png)

After:
![image](https://cloud.githubusercontent.com/assets/114601/4080722/33ce12e6-2ee1-11e4-9422-9a99fbfd4ed7.png)

The only thing I could go either way on is the width of the splitter (sash) bars.  I also think the "80% of the width of the parent" is a bit much, but I didn't want to affect too much.