Improve plugin interface

[leg0]

Currently, it is very difficult to store state of the plugin without resorting to global variables. To remedy this, the plugins should create an object, and return pointer to it to winlirc. The object is reference counted.

• removes need for init and deinit. when object is created, then the plugin is initialized, and when the object is destroyed, the plugin is deinitialized.
• no more global variables in plugins
• multiple instances of same plugin possible

IPlugin* createPluginObject();

...