ba4812 Added `Merge Levels (Do Not group Vector Levels)` and `Merge Levels (Group Vector Levels)` commands. Did not add them to the default menubar.

Authored and Committed by melieconiek 6 years ago
    Added `Merge Levels (Do Not group Vector Levels)` and `Merge Levels (Group Vector Levels)` commands. Did not add them to the default menubar.
    
    Added dialog box when merging vector levels via `Merge Levels` command--asks user "Group strokes by vector levels?".