Manipulate only primary cursor in multicursor mode

[goyalyashpal]

• Most of the time, the required mulicursor selection do have some common semantic/syntactic meaning,
  so can be done using same operations (moving by words, to end of line, etc.)
• But often time, say one of thosee selections is the exception / odd one out and requires additional tweaking
• ability to move only primary cursor will help a lot in those cases...

It can be thought of as having a primary cursor mode in multicursor.

[the-mikedavis]

This is something that should be solved with marks (#3720). With the current bindings on that PR you could start with multiple selections, save those selections to a register with save_selection_to_register (^s), use keep_primary_selection (,) to remove all non-primary selections, operate on that primary selection, and then use union_selection_from_register (^ca) to restore the other selections.