fset: contribute "light" theme overrides

Add fset-theme.{c,h} with the fset plugin contribution to the built-in
"light" theme: 47 overrides on fset.color.* options (line backgrounds,
selected-row tuning, title and value colors) tuned for a
light-background terminal.

The table is a NULL-terminated 2-column array of strings (no struct
wrapper, matching the pattern adopted for the irc contribution).
fset_theme_register builds a hashtable from the table and calls
weechat_theme_register; fset_theme_init() is called once from
weechat_plugin_init after fset_config_init / fset_config_read.

Default option values are NOT changed.

src/plugins/fset/fset.c

@@ -35,6 +35,7 @@
 #include "fset-info.h"
 #include "fset-mouse.h"
 #include "fset-option.h"
+#include "fset-theme.h"
 
 
 WEECHAT_PLUGIN_NAME(FSET_PLUGIN_NAME);
@@ -126,6 +127,8 @@ weechat_plugin_init (struct t_weechat_plugin *plugin, int argc, char *argv[])
 
     fset_config_read ();
 
+    fset_theme_init ();
+
     if (!fset_bar_item_init ())
         return WEECHAT_RC_ERROR;