core: implement /theme apply with themable enforcement and auto-backup

Implement /theme apply <name> for themes currently in the in-memory
registry. The file-shadowing branch (read a .theme file from
${weechat_config_dir}/themes/ when no built-in matches) is added in
the next commit together with the parser.

Apply algorithm (theme_apply in core-theme.c):

- Look up the theme in the registry; abort with an error if unknown.
- If weechat.look.theme_backup is on and the target name does not
  begin with "backup-", write a full snapshot of every themable
  option to ${weechat_config_dir}/themes/backup-<timestamp>.theme
  via theme_make_backup; abort the apply if the backup cannot be
  written, so the user can always undo.
- Iterate the theme's overrides with theme_applying=1 so the
  per-option config_change_color skips its gui refresh; for each
  entry look up the option, refuse it if missing or non-themable
  (warning to core buffer), otherwise call config_file_option_set.
- Perform a single gui_color_init_weechat + gui_window_ask_refresh
  at the end.
- Persist the active label in weechat.look.theme and send signal
  "theme_applied" with the name as data.

Add the new option weechat.look.theme_backup (boolean, default on)
which controls the backup-or-abort behaviour described above.

Wire the new /theme apply subcommand into core-command.c with the
existing /theme registration; update help text accordingly.

src/core/core-theme.h

@@ -44,6 +44,8 @@ extern struct t_theme *theme_search (const char *name);
 extern struct t_theme *theme_register (const char *name,
                                        struct t_hashtable *overrides);
 extern struct t_arraylist *theme_list (void);
+extern int theme_apply (const char *name);
+extern char *theme_make_backup (void);
 
 extern void theme_init (void);
 extern void theme_end (void);