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'
Remove an unnecessary allocation
Unify color parsing

Change utils_parse_color() to use gdk_color_parse() and follow its
syntax, additionally supporting our "0x" prefix as a synonym for the
"#" prefix;  and use this everywhere.

Also add utils_color_to_bgr() and utils_parse_color_to_bgr() to provide
conversion to the 24 bits BGR format used by Scintilla.
Remove useless utils_invert_color()
Remove useless rotate_rgb()
Remove unnecessary allocation for VTE color settings
VTE: properly use the default value for invalid configured colors
Plug a few possible small memory leaks
Autotools: enable Automake warnings
Autotools: always put object files in the source file's directory

Enable Automake option "subdir-object" to put all object files next to
their source file rather than in the directory containing the Makefile.

This is quite expected, and will be the future behavior of Automake in
all cases, so enabling it now avoids future unexpected change.
Autotools: fix building the documentation in VPATH builds
Autotools: use the target file as the base rule for building documentation

This allows automatic building of the file when required, and allows
user to request building that specific file.
Fix a typo in the documentation's markup
Fix truncation of the data retrieved with --list-documents

Do not truncate the document list to the size of the buffer used for
communication.

Since we now read multiple times, we need to make sure to always send
ETX so a reader don't hang reading the next chunk if the data sent had
exactly the size of the buffer.
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