added all BD engine elements shared with R'n'D engine as native elements

added all BD engine elements shared with R'n'D engine as native elements

There were a number of game elements covered by the native BD engine
that already existed in the R'n'D engine for a long time. So far,
these BD style game elements were "shared" between both game engines,
which had some implications and unwanted ambiguities (like defining
explosion animations which are played with different duration in both
game engines, resulting in graphical glitches, or differences in
game element properties which also requires different descriptions in
the level editor, which would simply be impossible with "shared" game
elements).

The following game elements had to be duplicated for the BD engine:

- bd_wall         -> bd_wall_native
- bd_rock         -> bd_rock_native
- bd_diamond      -> bd_diamond_native
- bd_magic_wall   -> bd_magic_wall_native
- bd_butterfly    -> bd_butterfly_1
- bd_firefly      -> bd_firefly_1
- bd_amoeba       -> bd_amoeba_1

This change does not make game element token names for artwork config
files nicer, but makes sure that there are no unwanted side effects as
described between both game engines anymore.