core: implement theme file parsing and transient file reads in /theme apply

Add a small INI-style parser for *.theme files and wire it into the
/theme command so user themes living in directory "themes" inside the
WeeChat configuration directory can be applied (and inspected) without
ever being cached.

Parser (theme_file_parse in core-theme.c) accepts two sections:

  [info]
  name = "..."          \ shown by /theme info; ignored for apply
  description = "..."   |
  date = "..."          |
  weechat = "..."       /
  (unknown keys are ignored with a warning)

  [options]
  full.option.name = "value"

Surrounding single or double quotes around a value are stripped (same
rule used by the regular config file reader). The parsed result is a
heap-allocated t_theme; the caller frees with theme_free.

Resolution rule in theme_apply: if the path
"${weechat_config_dir}/themes/<name>.theme" is readable it is parsed
and used (file shadows any built-in of the same name); otherwise the
built-in registry is consulted. The transient t_theme is freed before
the final refresh, so user themes have no steady-state memory
footprint regardless of how many .theme files have accumulated.

/theme list now also scans the themes directory and appends user
files to the listing (each marked "(file)"). backup-*.theme are
hidden by default; pass "-backups" to include them.

/theme info <name> works for both sources: file path is shown when the
information comes from disk; "built-in (in-memory)" otherwise.

src/core/core-theme.h

@@ -46,6 +46,9 @@ extern struct t_theme *theme_register (const char *name,
 extern struct t_arraylist *theme_list (void);
 extern int theme_apply (const char *name);
 extern char *theme_make_backup (void);
+extern char *theme_user_file_path (const char *name);
+extern struct t_theme *theme_file_parse (const char *path);
+extern void theme_free (struct t_theme *theme);
 
 extern void theme_init (void);
 extern void theme_end (void);