Add auto import and auto complete hint support

[georgema1982]

It would be better if auto import and auto complete hint is supported in salsa editor like the Java editor does. Right now when I type a class and hit ctrl+space, no hint is provided. I hope that, like Java editor, a list of available types are listed and when one is selected, it's also added to the import statements.

[angelozerr]

Same issue than #22