CMS-2972 Improve api.util.Loader

* moved loader classes to api.util.loader package
* created special loader events and put into the api.util.loader.event package
* create basic class BaseLoader
* refactored all loader classes to extend BaseLoader
Merge branch 'master' of github.com:enonic/wem-ce into CMS-2972

Conflicts:
	modules/wem-webapp/src/main/webapp/admin/common/js/form/inputtype/content/ContentSummaryLoader.ts
CMS-2972 Improve api.util.Loader

* moved loader classes to api.util.loader package
* created special loader events and put into the api.util.loader.event package
* create basic class BaseLoader
* refactored all loader classes to extend BaseLoader
Merge pull request #581 from enonic/CMS-2972

CMS-2972 Improve api.util.Loader