Bug 172072 - $title doesn't work in LateX header

In case $title is used in the latex header or footer it is now replaced with the empty string (it was left inside, resulting in non translatable LaTeX code), the documentation has been updated too.

See also:
Bug 668004 - LATEX_HEADER should share common information with HTML_HEADER
Bug 401327 - LATEX_HEADER: $title does not get expanded
Merge pull request #157 from albert-github/feature/bug_172072

Bug 172072 - $title doesn't work in LateX header