Read filetype's mime type from the configuration files

This allows custom filetypes to define a mime type, hence the icon
displayed for this filetype;  as well as moving ones for built-in
filetypes out of the code to the configuration files.
Use GIcon for the filetype icon rather than rendered pixbuf

This allows to re-use the icon more easily since it's not rendered at
a particular size but simply an icon description.  It also allows for
implicit icon updating when the theme changes if the display code
implements it (and GTK widgets does).
Fix crash in fallback mime icon loading code under GTK3
R: fix invalid memory access in the tag parser
R: properly display sources and libraries in the symbol list
Fix folding of the very last line in a level when unfolding children

Fix an off-by-one mistake resulting on the very last line of a nested
folded level not to be unfolded when recursively unfolding it's parent.

This was only visible when the last fold point was only one line long,
otherwise unfolding the (N-1)th line was enough.

Closes #1007.
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