Add new XCF property FLOAT_OPACITY and always save both the old 8-bit
and the new float opacity of layers and channels. Float opacity is
saved after the 8-bit one so when loading, it overwrites the limited
8-bit value with the proper precision. Do not increase the XCF version
number because old GIMP versions will simply skip the unknown
FLOAT_OPACITY and keep using the 8-bit value.