Elvis to If-Then, If-Then to Elvis intentions

Added an intention to convert an expression of the form

    foo ?: bar

to the form

    if (foo != null) foo else bar

and vice versa.

If the expression of the left hand side of the elvis expression is a function call, the IDE will create 
a temporary value to store the result of the call, and prompt the user for a name of the value.
i.e an elvis expression of the form:

    foo() ?: bar

could be converted to the form

    val temp = foo()
    if (temp != null) temp else bar