PLUGIN: Multiple - De-hardcode throughout

[Danweel]

For OP9, OP10, OP13, OP14, define these as constants at the top:

# --- CONSTANTS ---
BRUSH_ERASER_POINT = "Eraser Point"
BRUSH_ERASER_STROKE = "Eraser Stroke"
LAYER_BODY_LINES = "[BODY] Lines"
LAYER_HIGHLIGHTS = "[HIGHLIGHTS]"
MATERIAL_LINE = "1 I LINE"

This is mostly for longevity:

• If you rename a brush in Blender, you have to find and replace it in the code.
• If you rename a layer, you have to find and replace it in the code.

This is what that would look like, in theory:

# In OP9:
bpy.context.scene.tool_settings.gpencil_paint.brush = bpy.data.brushes[BRUSH_ERASER_POINT]