Support NUnit Project Loader Extension

[rowo360]

We should continue to support the NUnit project extension as we did in TestCentric 1.X

• The user can install this extension and afterwards it's been listed in the extension dialog
• The file dialog provides a file filter for nunit project files
• The user can select one or more nunit project files
• After the nunit project file is loaded, tests can be executed as usual
• We don't support modifying NUnit project files within TestCentric

Part of #1262

[CharliePoole]

@rowo360 See some edits to the description I made