[LV2] inline plugin displays (Robin Gareus)

[Vladimir Sadovnikov]

My look towards the concept.
0. In general, the idea is cool but needs some fixes.
1. The idea to build-in the display as an plugin-extension interface is bad. The 
original concept of LV2 was to separate the UI and the DSP accorting to MVC 
concept. So we have to follow it.
2. I think that the better solution is to pass from the host the handle of the 
window/or context for drawing, not to get this context from the extension interface.
For example, on Linux it can be an X Window, on Windows - HWND etc...
3. This idea can be extended to make possible the visibility of some important 
controls on the mixer stripe. So we return back to the UI extension.
Conclusion: this can be implemented as the sort of UI, the class of UI could be 
called also 'thumbnail'.
Because it is the sort of UI, I think, we don't need the additional extension 
interfaces, just the implementation of thumbnail UI (with support of GTK2, GTK3, 
QT4, QT5, etc...) and proper binding to the plugin.

Hope my thoughts have been clear and helpful.