Enhancement: Improvements in remove dialog

A new remove dialog has been created by adding the thumbnail of the file to be deleted.
Also, when removing files in multiple selection, the number of elements that are going to be removed is displayed in the dialog.

https://github.com/owncloud/android/issues/4342
https://github.com/owncloud/android/pull/4348
https://github.com/owncloud/android/issues/4377
https://github.com/owncloud/android/pull/4404
