Add custum LDraw part/sticker

[Frost]

• Create a folder called "unofficial" in your LDraw directory.
  
• Place your sticker's .dat file in the "unofficial" folder.
  
• Open LPub3D and go to the "Preferences" dialog box.
  
• In the "Libraries" section, click on the "Unofficial Parts" tab.
  
• Click on the "Add" button and select the "unofficial" folder that you created in step 1.
  
• Your sticker should now be available in LPub3D.