Open implementation instead of declaration in interface when using open-declaration

[addiks]

When the user currently uses the open-declaration function on the method of an interface the method-declaration in that interface will be opened. Most of the time that is not what the user actually intends. What the user actually intends is to open the implementation of that method.

The plugin should detect the case when the user uses open-declaration with the method of an interface and instead looks for implementations of that interface-method. If it finds only one implementation it should open that implementation. If it finds multiple implementations, it should ask the user in a new window what implementation (including the declaration in the interface) it should open.

If the user actually intends to open the method-declaration in the interface and the implementation opens instead, he can still uses open-declaration on the interface-name in the implementing class.