[Tagging] Simplify building:part areas

[marc marc]

First tag the whole building without any "part".
Therefore, building:levels, height refers to the maximum level and 
maximum height of the building, not an average.
If a single closed way is enough to draw a building, you don't need 
multipolygon at this time.
For nearly all applications that don't use "part", only those tag will 
be used.

Then you can add "parts" to better describe the differences in level, 
shape, height, color, type of roof of the different parts of the 
buildings. Those tags will only be used by applications that 
understand/use "parts".