Log Message:
Update contao35 to 3.5.2.

* Add Serbian language files.

Version 3.5.2 (2015-07-24)
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### Fixed
Revert some of the PhpStorm code inspector changes (see #7937).

Version 3.5.1 (2015-07-24)
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### Fixed
Add a `StringUtil` class to restore PHP 7 compatibility (see contao/core-bundle#309).

### Fixed
Fix the `Validator::isEmail()` method (see contao/core-bundle#313).

### Fixed
Strip tags before auto-generating aliases (see #7857).

### Fixed
Correctly encode the URLs in the popup file manager (see #7929).

### Fixed
Check for the comments module when compiling the news meta fields (see #7901).

### Fixed
Also sort the newsletter channels alphabetically in the front end (see #7864).

### Fixed
Disable responsive images in the back end preview (see #7875).

### Fixed
Overwrite the request string when generating news/event feeds (see #7756).

### Fixed
Store the static URLs with the cached file (see #7914).

### Fixed
Correctly check the subfolders in the `hasAccess()` method (see #7920).

### Fixed
Updated the countries list (see #7918).

### Fixed
Respect the `notSortable` flag in the parent (see #7902).

### Fixed
Round the maximum upload size to an integer value (see #7880).

### Fixed
Make the markup minification less aggressive (see #7734).

### Fixed
Filter the indices in `Database::getFieldNames()` (see #7869).

### Fixed
Back-ported two fixes from the upstream versions.