Constify metadata functions. Constify other functions that really needed
it. Make variables keeping old state mutable for now. Move filter composition
operations into biquad.h. Replace global function get_all_plugins with
a singleton class that returns the same list (as const) or a single plugin
by URI. Add a skeleton implementation of functions in lv2_external_ui
and skeleton UI related functions for external UI.
(cherry picked from commit ae7f7293961a3ea3b194b73ab35886f94e562cd7)
4a59150d4b