core: add themable flag to configuration options

Add an "int themable" field on struct t_config_option. The flag is set
automatically for every CONFIG_OPTION_TYPE_COLOR option, and may be set
explicitly on any other type by suffixing the type argument with
"|themable" in the call to config_file_new_option (e.g. "string|themable"
for a string option whose value contains "${color:...}" references).

Opt in the relevant string options in core (buffer_time_format,
day_change_message_*, item_time_format, nick_color_force, prefix_*,
chat_nick_colors, eval_syntax_colors, color palette aliases) and in the
buflist, fset, irc, relay plugins.

The flag is exposed via hdata, infolist, and print_log so scripts and
/debug can read it. This is the foundation for an upcoming /theme
command that will only be allowed to modify themable options.

src/plugins/irc/irc-config.c

@@ -3959,7 +3959,7 @@ irc_config_init (void)
             NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL);
         irc_config_color_mirc_remap = weechat_config_new_option (
             irc_config_file, irc_config_section_color,
-            "mirc_remap", "string",
+            "mirc_remap", "string|themable",
             /* TRANSLATORS: please do not translate the list of WeeChat color names at the end of string */
             N_("remap mirc colors in messages using a hashtable (used only "
                "for standard colors, not RGB colors): keys are \"fg,bg\" as "
@@ -3977,7 +3977,7 @@ irc_config_init (void)
             NULL, NULL, NULL);
         irc_config_color_nick_prefixes = weechat_config_new_option (
             irc_config_file, irc_config_section_color,
-            "nick_prefixes", "string",
+            "nick_prefixes", "string|themable",
             N_("color for nick prefixes using mode char (o=op, h=halfop, "
                "v=voice, ..), format is: \"o:color1;h:color2;v:color3\" (if a "
                "mode is not found, WeeChat will try with next modes received "
@@ -4008,7 +4008,7 @@ irc_config_init (void)
             NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL);
         irc_config_color_term_remap = weechat_config_new_option (
             irc_config_file, irc_config_section_color,
-            "term_remap", "string",
+            "term_remap", "string|themable",
             N_("remap terminal color numbers in messages using a hashtable "
                "(used only for RGB colors as hexadecimal, which are first "
                "translated to terminal color numbers): keys are \"fg,bg\" as "