Use LV2 state to store preset index in presetBank, to unload the .ttl

[p0nce]

See also #808

Blocked by futureBinState not being default.

even without futureBinState, saving a state allows to use a whole Dplug chunk, hence not relying on host for resetting state... => I don't think future changes would be state-V1 incompatible? So I don't see a point in saving the whole plugin state in binary. Maybe more compact than .ttl port descriptions though? => this probably can be done later, as presets can change without breaking compat. If we do that, we'll use another URI instead of pluginState (say, pluginChunk = pluginState + parameters). Besides, it's not clear now how much LV2 hosts like our state interface.

[p0nce]

To unload the .ttl, another option is to just store an internal preset number in another URI.