menuselect: Add menuselect to Asterisk trunk (Patch 1)

This is the first patch that adds menuselect to Asterisk trunk, and removes
the svn:externals property. This is being done for two reasons:
(1) The removal of external repositories eases a future migration to git
(2) Asterisk is now the only thing that uses menuselect; as a result, there's
    little need to keep it in an external repository

Subsequent patches will remove the mxml dependency from menuselect and tidy
up the build system.

ASTERISK-20703


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@418832 65c4cc65-6c06-0410-ace0-fbb531ad65f3