gtk:bitset-splice (bitset pos removed added)

bitset -- a gtk:bitset instance
pos -- an unsigned integer for the position at which to slice
removed -- an unsigned integer for the number of values to remove
added -- an unsigned integer for the number of values to add

This is a support function for g:list-model handling, by mirroring the "items-changed" signal. First, it "cuts" the values from pos to be removed from the bitset. That is, it removes all those values and shifts all larger values to the left by removed places. Then, it "pastes" new room into the bitset by shifting all values larger than pos by added spaces to the right. This frees up space that can then be filled.