Fix 0003530: Custom fields definitions are not validated.

M core/constant_inc.php

• Added constant for new custom field error (ERROR_CUSTOM_FIELD_INVALID_DEFINITION).

M lang/strings_english.txt

• Added English string for new custom field error (ERROR_CUSTOM_FIELD_INVALID_DEFINITION).

M core/custom_field_api.php

• Added the following checks on updating of custom fields:
  • Name is not blank.
  • Name is still unique
  • access level for read/write is smaller than or equal to access level for read.
  • Minimum length >= 0;
  • Maximum length is 0 (disabled), or greater than or equal to minimum length.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2292 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9