Implement custom values for filteron=

Edit Subject

While the filteron command is usefull, it is also very limited
It has potential to allow for navtree customization that is not possible today

For Ex:

One could create a container of items for a specific folder like this

< container name='videos' id='video' >
< container buildon='res' filteron='res=folder name' / >
< container/ >

One could create a container of items for a specific genre like this

< container name='videos' id='video' >
< container buildon='res' filteron='upnp:genre=genre name' / >
< container/ >