move opds to plugin

[pazos]

Since it is not a required "app" like the reader or the filemanager. Also all the stuff now is placed together and can be disabled :)

1. replace hardcoded menu entry by menu hint.
2. added a dispatcher action to show catalog (from FM)

to-do: better name and description in _meta.lua

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This change is 

[Frenzie]

I suggested it in so many words, so what can I say? ;-)