Add support for exponentiation (using the ^ operator) to eevl.
This is occasionally useful, e.g., in geometrically-derived
quantities involving square roots.
Pass the evaluation options to gimp_eevl_evaluate() using a single
parameter of type GimpEevlOptions, instead of using individual
parameters for each option. Add a GIMP_EEVL_OPTIONS_INIT macro,
used to initialize a GimpEevlOptions struct to the default set of
options. This would allow us to add evaluation options more
easily.
Add a simple parser to the GimpSizeEntry widget so that one can write
things such as "40in" and "50%" in a size entry widget and get that
converted to the current unit.
The parser also handles basic expresions such as "20cm + 20px" and
"2 * 3.14in".
