core: add core-theme skeleton and theme registry

Introduce a new module (core-theme.{c,h}) holding the in-memory registry
of built-in themes used by the upcoming /theme command:

- struct t_theme stores name, description, date and weechat version
  captured at registration time, plus a hashtable of overrides keyed by
  full option name (file.section.option) -> value string.
- theme_register (name, overrides) creates a new theme or merges the
  given overrides into an existing one (later calls override duplicate
  keys); this is the API plugins and scripts will use to contribute
  per-theme color values.
- theme_search and theme_list provide lookup and ordered enumeration.
- theme_init / theme_end are called from weechat_init / weechat_end.

The theme_applying flag is declared here but not yet consumed (it will
gate config_change_color in the next commit to avoid N redundant
window refreshes during /theme apply).

User theme files are not handled by this module: they are read
transiently inside /theme apply (a later commit) and never cached.

src/core/weechat.c

@@ -75,6 +75,7 @@
 #include "core-secure-config.h"
 #include "core-signal.h"
 #include "core-string.h"
+#include "core-theme.h"
 #include "core-upgrade.h"
 #include "core-url.h"
 #include "core-utf8.h"
@@ -384,6 +385,7 @@ weechat_init (int argc, char *argv[], void (*gui_init_cb)(void))
         weechat_shutdown (EXIT_FAILURE, 0);
     if (!secure_config_init ())         /* init secured data options (sec.*)*/
         weechat_shutdown (EXIT_FAILURE, 0);
+    theme_init ();                      /* initialize theme registry        */
     if (!config_weechat_init ())        /* init WeeChat options (weechat.*) */
         weechat_shutdown (EXIT_FAILURE, 0);
     args_parse (argc, argv);            /* parse command line args          */
@@ -449,6 +451,7 @@ weechat_end (void (*gui_end_cb)(int clean_exit))
     unhook_all ();                      /* remove all hooks                 */
     hdata_end ();                       /* end hdata                        */
     secure_end ();                      /* end secured data                 */
+    theme_end ();                       /* end theme registry               */
     string_end ();                      /* end string                       */
     weeurl_end ();
     weechat_shutdown (-1, 0);           /* end other things                 */