Multilanguage Support via gettext

- added gettext encapsulations to all texts i thought necessary
- added fallback function for the case that the gettext extension isn't loaded
- added browser language detection
- added gettext to the list of optional extensions in setup
- rewritten some of the texts to use sprintf instead of appending strings
- added german translation file
- removed mark_overdue-confirm from cannedresponses.inc.php

extend multi language support and a fex fixes

- Better detection of translation files
- Added functionality to redirect language codes (see redirecting
language codes)
- Ticket Status can be translated
- The Datepicker can be translated
- Extended functionality of 'testlang.php' to show what language code is
used to translate

Forgotten to apply a patch from RC5 to RC6

- Forgotten to change $var to $vars in line 380 of class.mailfetch.php
- Removed unneeded comment

Added php_gettext as primary translation engine

- Added php_gettext support (thanks to Danilo Segan and Steven
Armstrong)
- php_gettext is now the primary translation engine
- Extended language detection functionality

Add error/misconfiguration checks and fix undefined variables

pjax:complete is only called after an AJAX page load. If a user navigates
forward or backward and the page loads from local cache, then pjax:complete
is not fired. pjax:end, however, is always fired.

Add original and last message quotes to canned responses

Otherwise, no column will be added to the %ticket__cdata table and the
ticket queue pages will be crashed (empty).

The default mode going forward will be to insert when Rich Text support is
enabled and append otherwise. To clear the draft the user can simply click
reset or delete draft icon on the bar.

Enable delete draft option on reply & internal note boxes for consistency.

If the advanced search dialog is up, avoid refreshing the page

htmLawed stripped/discarded <div> tags nested inside inline tags such as
<a>. The previous replacement engine would replace <img> tags with <div>
tags and would cause the HTML to be corrupted by htmLawed.

This patch uses <span> tags which have CSS rules to be displayed as
inline-blocks. This should stay truer to the normal sizing and placement of
the original <img> tag.

And use admin-defined site logo

This reverts commit b6b2b857.