Improve the way how get_region_name() method check for string existence

At the moment, the theme_config::get_region_name() checks if the first character of the get_string() is a square bracket to interpret it as whether the string exists or not.

This is not right, even if it works.

• The fact that non-existing strings are returned in certain format starting with the bracket, is kind of side debugging effect. The value is really undefined and should not be relied upon.
• The string could actually exist and be worded so that its first character is also the square bracket.

There is a string API method string_exists() that should be used instead.