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
Add some whitespace to resolution tree labels

Signed-off-by: Sean Bright <sean@malleable.com>
Merge pull request #904 from seanbright/resolver-ui-whitespace

Add some whitespace to resolution tree labels
Merge pull request #900 from seanbright/release-ui-cleanups

Minor cosmetic changes to the Release UI
Add CONTRIBUTING documentation

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Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #906 from bjhargrave/contributing

Add CONTRIBUTING documentation
Update README to mention new CONTRIBUTING document

[ci skip]

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #907 from bjhargrave/readme

Update README to mention new CONTRIBUTING document

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