FR: Allow partial solving of conflicts using built-in merge editor

[TotNotTac]

Is your feature request related to a problem? Please describe.
Sometimes merge conflicts can only be partially solved using a simple conflict resolution tool like the built-in merge editor. In this case some hunks will require manual editing using a text editor (think of reordering of statements, or combining imports, etc). In the current situation, the choice for manual resolution has to be made before you can start resolving the conflicts, since the built-in merge editor can only mark hunks as either wanted or unwanted, and not as unresolved.

Describe the solution you'd like
The built-in merge-editor should have an option to mark areas as unchanged or unresolved, so they can be solved manually at a later time.

Describe alternatives you've considered
Use another merge-editor than the built-in one.