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
Merge pull request #900 from seanbright/release-ui-cleanups

Minor cosmetic changes to the Release UI