[Theme] Try to fix importing of theme.

- Fix the two place of resolving into one method.
- Do not accept file in CWD.
- Prefer file if it exists in same directory as parent file that
  imported it.

fixes: #1889

include/helper.h

@@ -397,8 +397,9 @@ char *helper_string_replace_if_exists(char *string, ...);
  *
  * @returns path to theme or copy of filename if not found.
  */
-char *helper_get_theme_path(const char *file, const char **ext)
-    __attribute__((nonnull));
+char *helper_get_theme_path(const char *file, const char **ext,
+                            const char *parent_dir)
+    __attribute__((nonnull(1, 2)));
 
 /**
  * @param name The name of the element to find.