Better expose the feature: Invert condition

[jdneo]

Currently the Invert Condition refactoring can only be triggered if the user selects a valid boolean expression, which is not very convenient to use.

We can better expose it actually:

• If the selection in context is more than 0, we follow the current implementation.
• If the selection in context is 0, we can try to find the highest boolean expression node in the AST Tree which contains the covering node.

I can send out a PR for this later.