Add key bindings
I think we must decide which keys to bind for language specified features, such like

* Fix imports.
* Show documentation.
* Show variables usage (in file/in project).
* Go to previous/next method
* Go to implementation.
* Go to type declaration.
* Go to super class (or method).
* Go to test class (or method).
* Rename variable.
* Fix linter errors.
* Place breakpoint.
* Start debugging.
* Override interface methods.
* Generate constructor/getter/setter/getter and setter/equals/test method

Once these key mappings are defined, contributors will be able to implement them easily.