Rewrite of dw_window_destroy() on GTK2 so that the table contracts removing empty cells when destroying child widgets. This keeps the indexes correct for packing later. Similar in functionality to the last commit for GTK3... but uglier although it works better because the GTK3 fix just leaves the empty cells and skips over them... this actually removes the cells.

# Set these here to change the version for all platforms
DW_MAJOR_VERSION=2
DW_MINOR_VERSION=3