GPv3: Draw lines between frames

Draws lines between keyframes indicating that they are held.
This is similar to legacy grease pencil.

Pull Request: https://projects.blender.org/blender/blender/pulls/125813

source/blender/editors/animation/keyframes_keylist.cc

@@ -530,6 +530,7 @@ static ActKeyColumn *nalloc_ak_cel(void *data)
   /* Set as visible block. */
   ak->totblock = 1;
   ak->block.sel = ak->sel;
+  ak->block.flag |= ACTKEYBLOCK_FLAG_GPENCIL;
 
   return ak;
 }