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.

tests/unit/tests.cpp

@@ -77,6 +77,7 @@ IMPORT_TEST_GROUP(CoreNetwork);
 IMPORT_TEST_GROUP(CoreSecure);
 IMPORT_TEST_GROUP(CoreSignal);
 IMPORT_TEST_GROUP(CoreString);
+IMPORT_TEST_GROUP(CoreTheme);
 IMPORT_TEST_GROUP(CoreUrl);
 IMPORT_TEST_GROUP(CoreUtf8);
 IMPORT_TEST_GROUP(CoreUtil);