Rework clientlib permission checks

[stoerr]

The client library sling console plugin now prints for the impersonation user:

• informational message what client libraries are unreadable
• errors when a client library is readable but one of it's elements is not
• errors when a client library references unreadable client libraries or files
• errors when something is referenced from a client library but not present in JCR

[stoerr]

BTW: there are the following broken references discovered by the checker. I didn't fix that so far, since I'm not sure whether these are unnecessary since nobody missed them so far:
( http://localhost:9090/system/console/clientlibs?type=all&lib=&impersonate=admin )

ERROR: can't find element jslibs/bootstrap/3.3.5/js/bootstrap.min.js of resource folder /apps/composum/prototype/themes/solid/clientlib/body/js
ERROR: can't find element jslibs/bootstrap/3.3.5/js/bootstrap.min.js of resource folder /apps/composum/prototype/themes/solid/clientlib/body/js
ERROR: can't find element composum/pages/components/clientlibs/page of resource folder /apps/composum/prototype/pages/documentation/clientlibs/page/css
ERROR: can't find element composum/pages/components/clientlibs/page/body of resource folder /apps/composum/prototype/pages/documentation/clientlibs/page/body/js
ERROR: can't find element composum/pages/components/clientlibs/page/body of resource folder /apps/composum/prototype/pages/documentation/clientlibs/page/body/js
ERROR: can't find element fonts/awesome/4.6.1/css/font-awesome.min.css of resource folder /apps/composum/prototype/pages/documentation/clientlibs/page/css/fonts
ERROR: can't find element composum/nodes/browser/js/nodes.js of resource folder /libs/composum/assets/manager/clientlibs/manager/js
ERROR: can't find element fonts/awesome/4.6.3/css/font-awesome.css of resource folder /libs/composum/pages/wiki/clientlibs/page/css/fonts
ERROR: can't find element composum/nodes/console/page/js/console.js of resource folder /libs/composum/nodes/console/clientlibs/codeeditor/js