Rename utils_strtod() to utils_parse_color()

Rename the function to a more sensible name, and remove unnecessary
arguments.  Now, format (# or 0x) is auto-detected.
Properly validate input in utils_parse_color()
Accept colors with only 1 digit per channel in utils_parse_color()

AKA short HTML color notation.
Simplify reading highlighting colors with new utils_parse_color()

Drop redundant code converting # and short formats to hex colors now
utils_parse_color() can parse them.
Simplify fold toggling code using higher-level Scintilla API
Merge branch 'wip/color-parsing-improvements'