CORE-4383 Updates to AppsView and Presenter for Drag-n-Drop support.

Updated AppsViewImpl#tree and grid to protected so that Belphegor has access to them for Drag-n-Drop support.
Added getAppGroupFromElement and getAppFromElement methods to AppsView and AppsView#Presenter, required for Drag-n-Drop support.
CORE-4383 Updates to allow Groups to reload in Belphegor.

Changed AppsViewPresenter#initHandlers to protected, so Belphegor can initialize its group refresh handler there.
Added an AppsViewPresenter#reloadAppGroups method so that groups can be refreshed, and added a 'loading' mask during group loading.
Added an AppsView#clearAppGroups method for the new reloadAppGroups method.