... as I would like it to.

Use GimpSpinButton, added in the previous commit, in GimpSizeEntry,
instead of GtkSpinButton.  This avoids updating the spin-buttons'
adjustment values when they lose focus, truncating the value if it
can't be accurately displayed using the corresponding spin-button's
digit count.  Since size-entries can have multiple spin-buttons
using different units, this prevents the value from changing when
entring a value using one unit, and then shifting the focus to, but
not changing, another unit.