Workaround for EXTJS-15054 in ExtJS 4.2.3

After upgrading to ExtJS 4.2.3 an exception was thrown in PDF.js:

  Uncaught TypeError: Cannot read property 'isItemBoxParent' of undefined

Following the hints on the forum, moving LoadMask from afterrrender to boxready solved the problem.

http://www.sencha.com/forum/showthread.php?291412-Error-after-upgrade-to-ExtJS-4.2.3&p=1064679