Beginning to develop a solution for issue 14729

As suggested by aphadnis, I invented a setting to determine the issue attribute on which the coloring in the issue list is based on (instead of always coloring by status). It works and required only few changes so far but only priority is supported right now.

Because I am new to Mantis, I thought it might be a good idea to open a pull request and have a bit of discussion before I continue. :)