Added failsafes for retrieving remote apps on settings->apps page, in case categories or app data are not found

Added method getAppstoreApps to OC_App
Added minor documentation

LDAP: fix MySQL-query with DUAL table by removing wrong backticks. Got broke when they were added to SQL queries.

Fixes: oc-1369

This didn't work with chunked uploading

This should also fix the dirname problem on windows

fix default values: 1. remove whitespace, don't use emptystring for int, remove empty default for nullable columns to use NULL as default

every translation within the sub folder 'settings' need to use 'settings' on OC_L10N::get

i just learned that we also have to check if directories are readable. it seams that is_readable also checks for 'x' right.