Re-implement URIMap to tolerate broken plugins that use the wrong context to

Re-implement URIMap to tolerate broken plugins that use the wrong context to
map MIDI event types (fix #4889).

All uri-map contexts are now just ignored, and equivalent to urid (which is
equivalent to uri-map with context NULL).  We now just hope that no event types
are mapped after UINT16_MAX URIs have been mapped, and die horribly otherwise.
This is exceedingly unlikely to happen any time in the next several years, if
ever.

git-svn-id: svn://localhost/ardour2/branches/3.0@12462 d708f5d6-7413-0410-9779-e7cbd77b26cf