add smart completion for ValueMapping

[hduelme]

As discussed in #5 I add a smart filter to prevent already mapped source values from being displayed.

@filiphr could you have a look?

[filiphr]

Thanks for this PR @hduelme, can we please add some tests for this?

It would also be good not to use Impl classes such as PsiLiteralExpressionImpl, I think that there is PsiLiteralExpression.

[hduelme]

@filiphr I have replaced the usage of the Impl class by PsiLiteralExpression. I also added 2 tests:

• one source value already mapped
• all source values already mapped

[filiphr]

Thanks @hduelme. I've integrated this in https://github.com/mapstruct/mapstruct-idea/commit/2a80757c06ed6e7725dd140f6abe4e6eb058b414