[UI Icons] Add the many missing icons and replace the placeholder ones

[Kranos_Orphydian]

The Icon work that has been done by the Max UI team is one of the most rock solid ones and possibly the most decent one in any 3d software. There are still many icons missing, especially in the menus, and a few placeholder ones (like the one for the new Spline modifiers) that would need this great work to continue.