Explicitly allow not giving a relative path.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@33272 a95241bf-73f2-0310-859d-f6bbb57e9c96

Less hardcoding of paths. Adding shared private OpenWithTracker().

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@33207 a95241bf-73f2-0310-859d-f6bbb57e9c96