Several types functions were using the wording "float" historically to
mean double-precision, e.g. the float array type (which was in fact a
double array). Or the scanner function gimp_scanner_parse_float() was in
fact returning a double value. What if we wanted someday to actually add
float (usually this naming means in C the single-precision IEEE 754
floating point representation) support? How would we name this?

Now technically it's not entirely wrong (a double is still a floating
point). So I've been wondering if that is because maybe we never planned
to have float and double precision may be good enough for all usage in a
plug-in API (which doesn't have to be as generic so the higher precision
is enough)? But how can we be sure? Also we already had some functions
using the wording double (e.g. gimp_procedure_add_double_argument()), so
let's just go the safe route and use the accurate wording.

The additional change in PDB is internal, but there too, I was also
finding very confusing that we were naming double-precision float as
'float' type. So I took the opportunity to update this. It doesn't
change any signature.

In fact the whole commit doesn't change any type or code logic, only
naming, except for one bug fix in the middle which I encountered while
renaming: in gimp_scanner_parse_deprecated_color(), I discovered a
hidden bug in scanning (color-hsv*) values, which was mistakenly using a
double type for an array of float.