Replace [trac] mainnav and [trac] metanav with order attributes of the corresponding sections

[Ryan J Ollos]

Since we have [mainnav] and [metanav] sections in trac.ini, it seems odd to have the order of the entries still determined through the [trac] mainnav and metanav options. Instead we could add an .order attribute to each option in the [mainnav] and [metanav] section.

This might also give us a way to address the issue of the names of the items being undocumented, particularly for plugins (​gmessage:trac-users:rGMQ0XPRsww/8Rm85C0F8wQJ). Perhaps all the navigation items provided by plugins could be written to the section with a .order attribute. This might somehow be related to #11437.