Enabling possibility to have { and } in (latex) index items

In the doxygen manual the index items for \{ and \} were missing due to the missing support for the usage of { and } in parts of the code.
This patch fixes this problem by introducing 2 new latex commands. See also http://tex.stackexchange.com/questions/153291/index-unmatched-braces-in-latex
Further improvements in the index are:
- consistency in different places
- correction of index for \::
- placing \~ on a more logical place (together with other special characters, ~ is in the ASCII table after a-z whilst other characters are before this range)