Change GtkAdjustment to double

[Owen Taylor <otaylor redhat com>]

I'd like to go ahead and change GtkAdjustment to use doubles not
floats. 

Yes, usually, it doesn't matter much, but the fact that 32-bit
integers dont' have exact representation as doubles makes for
a trap that simply don't need to have.

Plus, almost everywhere else in GTK+ and and the standard C library,
doubles are used as the generic floating-point-number representation.

So, if you object, whine quickly! ;-)

                                        Owen