LV: refactor lv_plugin_* API

[rigid]

• improve version check to better judge compatibility (like, "plugin matches API", "plugin older but working", "plugin too old to load" outputting deprecation warnings etc.)
• simplify code
• ...

[davidwe]

What about modeling a plugin as an object oriented abstract class or interface?

[kaixiong]

Yup that is exactly the idea :)

[rigid]

Depends on #26