Fixed #4043 3.1 Remote API - client_add: sys_user insert fails if client parameter 'usertheme' is empty

- Add option to _get functions of the remote-api to return all records when primaryID = -1
- Fixed permission problem in _get functions of remote api.
- Fixed typo in german dashboard language file.

Fixed: FS#3226 - mail_user_update FAIL for changes with quota >2048 MB (Soap Error: quota_error_isint)

- Added: First version of client api access with client user credentials
- Changed: 
  - merged remoting_lib and tform into one new base file where appropriate
  - moved some methods into helper methods
  only those methods remain in remoting_lib or tform that are completely
  different in these files or are only used in one of them
- Changed: added check of _GET array in json api handler

- need to have the pecl idn and intl modules installed
  or
- have the idna_convert class in the classes/idn/ path

Fixed/Implemented: replaced intval() by $app->functions->intval() in all interface functions due to big number problem in intval()

- Implemented: FS#1935 - Add minimum allowed seconds Value for DNS zone records

Bugfix: remoting lib field validation "ISINT" differed from tform validation and leads to 0 values treated as invalid
Bugfix: remoting lib did include_once instead of include at form definition files, that leads to malfunction when using multiple definition files at importing and actions like that