GPencil: Move Object Depth Order to Object

There was a conflict between Object "In Front" and how the strokes were managed in Stroke panel for grease pencil datablock.

In order to keep consistency, the order of the object has been moved to Object level and now it's using the standard "In Front" flag. As all it is 3D, the option "Back" has been removed.

This also fixes the problem with task T62137.

Also related to commit 54ffc4e19d and D4405

source/blender/blenkernel/intern/gpencil.c

@@ -411,7 +411,6 @@ bGPdata *BKE_gpencil_data_addnew(Main *bmain, const char name[])
 	/* GP object specific settings */
 	ARRAY_SET_ITEMS(gpd->line_color, 0.6f, 0.6f, 0.6f, 0.5f);
 
-	gpd->xray_mode = GP_XRAY_3DSPACE;
 	gpd->pixfactor = GP_DEFAULT_PIX_FACTOR;
 
 	/* grid settings */