add material type selection for choosing material presets

actual implemented material types are:
- glass
- wood
- ice
- rubber (interestingly results in negative compression values o.o)
- undefined (is automatically set, if preset values are changed)

main.cpp uses glass as first chosen material. you can choose another material presets via the dropdown menu. this will adjust the values for elasticity and compression. elasticity and compression are given in GPa, thus values will be multiplied with 1E9 when the material is being recalculated.

update_grid_forces does no longer calculate all physics values from E and K. the lame constants are calculated in the material itself. so, the physics structure does only contain time parameters and the lame constants, else the other values would be redundant.