Update CONTRIBUTING.md

Add tracker URI and a request to keep issues on topic.
Fix gamut mask template dialog exception

Fix an exception when trying to use the New button in the gamut mask
dialog. Looks like the shape-builders stanzas got typo'd in some of the
recent code cleanup commits.
Added a note on build-dep

Running `sudo apt-get build-dep` is a lot more convenient, but won't
work well if packages are too old.