Revise the patch to refactor proposals logic into jdt.core.manipulation

- split the proposal logic into core and U moving the vast majority of the logic into the core classes
- maintain existing UI non-core classes as wrappers for stability.
- create IInvocationContextCore class and makes it the default interface for use in the core classes
- mark newly split packages with mandatory directives (#833)

 
Signed-off-by: Rob Stryker <stryker@redhat.com>
Co-authored-by: Rob Stryker <stryker@redhat.com>
Co-authored-by: Jeff Johnston <jjohnstn@redhat.com>
Co-authored-by: Andrey Loskutov <loskutov@gmx.de>
Co-authored-by: Thomas Watson <tjwats@gmail.com>