refactored EntitySelector DOM manipulation internals

ideally this component should not be aware of Bootstrap in the first
place, but that's a different issue
refactored EntitySelector internals to avoid spurious instance variables
refactored indicator handling within EntitySelector

avoids excessive instance variables and improves readability

in the process, removed obsolete class (cf.
bd93ea631dd7fdd75dc07e9351d281264efc27cc)